By firearms, does the OP mean only human-portable guns, or does heavy artillery like mortars, tank cannons, howitzers, etc. all fall into the category of “firearms?”
If all the latter, then it would change warfare considerably during the few months of time that it takes the guns to come back, although bombs, missiles and whatnot would still play a big role. Incoming waves of enemy infantry might be a lot harder to stop.
If all firearms magically disappeared, the little safe that holds my pistol would instead hold my family’s passports. (I have a lockbox for them currently, but the safe would be better.)
I’d also be annoyed because that gun cost quite a bit when I got it.
I do admit, I’d like to see what happens on the Russian/Ukrainian front if this scenario happens. Sure, Russia has a LOT of artillery, but their “security” troops would suddenly have a LOT less to threaten with. I think it would be game-changing in the local theater, but since we know Putin will bomb and gas his own people to win, I’m not sure he’d end up tossed from power.
Airguns would see a huge increase in sales / production / ownership, largely (but not nearly completely) filling the void. After all, there are airguns today that can be used to kill dangerous large game, or to kill people easily at 100 yards (power + accuracy).
More like crossbows. Few airguns can kill a human, more like a rodent. Mind you they do make special airguns that are powerful, but they are rare.
No. Even the medium- and higher power spring-powered airguns can be and have been used to kill people inside 40 to 50 yards, even with single shots to the torso. There are literally millions of these guns already out there, and they can be purchased for 100 to 400 dollars at thousands of retail facilites / sites. A plus here is that the ammo is very cheap compared to firearm ammo.
If firearms vanished, the production of the highest-power PCP airguns (these are the ones that can be used to hunt deer, boar, and even cape buffalo) would skyrocket almost overnight, to fill the void. They would become very common.
Crossbows are mostly fantasy fodder. I’ve had a number of them and hunted with them. They combine the downsides of archery, namely low velocity / high trajectory to a very low rate of fire, being single-shot and the most awkward hunting tool there is to load. They are also hard to maneuver either in the woods or inside buildings, having a substantial horizontal component to them, which firearms and airguns have not.
To a firearm nation, airguns are infinitely more adoptable by the general populace than crossbows, even though the latter can be extremely lethal and quite accurate, at short to medium distances.
There aren’t very many airguns compared to firearms, and not very many airgun factories compared to firearm factories. If all of the firearms vanished, then the people who already had airguns might count themselves fortunate, but anyone who didn’t already have one would have to wait for one to be manufactured, at which point they might as well just wait for a firearm to be manufactured.
There are even single airgun models with production numbers up to 2 000 000. There are millions and millions of airguns around. Was it like 400 000 000 civilian firearms across the U.S.? Most countries in the world aren’t like that. Are we restricting ourselves to the U.S. in this hypothetical? Many countries with highly restricted firearm ownership have a healthy airgun ownership rate.
As it is, airguns compete with firearms, and in countries such as the U.S., where civilian firearms can be easily purchased, people choose the latter, for the many advantages they have over airguns. If firearms vanished, the airgun manufacturers would see a huge increase in sales. PCP airguns aren’t hard or expensive to make - the production could be ramped up fast.
These days, new airgun manufacturers pop up like mushrooms after rain, even without any magical help, but I don’t think many at all are in the U.S. This probably skews the view for some.
Crime would go way down.
Militaries would replace their guns much more quickly than warlords, so the power of the nation state — example Mexico — would increase. The effect from one country to another would vary, but it would be positive for years.
As for the idea that the missing guns could be quickly manufactured, maybe. But a lot of the guns involved in crime and warlordism are illegally obtained. Gun smuggling is a lot harder and slower than authorized distribution.
Also, when people saw the wonderful results of the miracle, they would want stronger gun control. In the U.S.? Maybe not, but I wouldn’t focus on one country that is so atypical in its gun culture.
Please God, consider this.
My understanding is that crossbows at one time filled a niche between traditional bows and firearms. The problem with archers is that it took a lifetime to be really good at it, enough to be effective on a battlefield. In Medieval Europe at least, your armies would largely comprise of conscripted peasants that you could hand polearm weapons to, have them march in formation, and hack at an enemy. But you couldn’t pull a farmer out of a field and hand them a bow, you needed someone who was a professional for that.
Crossbows didn’t require nearly as much training. They had all the downsides you list, but as an intermediary range weapon they worked well enough and could be used by conscripts.
Once gunpowder became available though, it took the place of crossbows, and when firearms were more accurate and efficient they took the place of all weapons for the most part, at least in war.
I agree with this. I do wonder how tentative people would be to make firearms though. How do they know it won’t happen again? It might make some people hesitate to invest much (manufacturers, sellers, and buyers alike) at first.
Yes, this is general knowledge. I’m not quite sure what about my reply you addressed with this?
The fantasy fodder moniker I used was in today’s context, with very long-range, very accurate, very easy to shoot and very quick to load firearms in common use.
Crossbows were effective battlefield and hunting weapons when firearms were in their infancy. They had a number of advantages as well as substantial disadvantages over hand held bows.
I was agreeing with you. Crossbows had a relatively brief period of relevancy and I don’t think that would be the case again just because firearms vanished. There are other options now.
OK, cool.
To be fair, with advances in materials and engineering since the middle ages, modern crossbows are now much better than their medieval predecessors.
But still way behind firearms. The only reason anyone ever uses a crossbow any more is that some states have a special hunting season for them.
Small nitpick with the above (both your and @Toxylon’s general points and discussion as their relevance to this thread are fine though!) - while they would indeed have been easily used by conscripts, that’s not how they were used, because they were comparatively expensive, or at least the ones made with iron/steel during the European middle ages. So, more specialty troops, or mercenaries, rather than conscripts who could be given little more than a spear and yet still be of use.
And we’re leaving our huge periods of time in which they were in favor in the early developmental period of China, although those bows included wood and bronze bowed options, as well as repeaters.
So, like everything, nuanced. 
But, and back to this thread, yes, modern crossbows are easy to use in a general sense, and certainly compared to any sort of longbow, but outside of niche (and mostly fictional) scenarios where reusability of bolts or silence becomes key, almost any of the alternative’s mentioned upthread are a better idea.
That’s fair, yes. It’s still true that they don’t need to be used by specialists like a traditional bow, but the expense of a crossbow isn’t something you’d trust to a nobody. Early firearms wouldn’t have been either I’m sure.
That’s why I qualified my statement by saying it was the case in Medieval Europe. Things happened very differently in other parts of the world.
Heck, if my pistol vanished, maybe I’d consider a chu-ko-nu to replace it… 
Why wouldn’t they just make more real guns?
Right.
Lots of people hunt with them, sure the RoF is low, but the power can be pretty high.
I doubt that. Murder would go way down, sure, at least for a while.
https://worldpopulationreview.com/country-rankings/crime-rate-by-country
USA has an overall crime rate of 49.2, the UK, 46.9. And pretty much handguns are banned there.
Venezuela has a crime index of 83.76, the highest of any country in the world. And it has very strict gun laws. You dont need guns to rob people. a knife will do nicely.
But there are a good number of them- and bows- out there.
I have a good number of swords…
Just look at the Musketeers: The only folks who got firearms were the guys who were already the elite fighting force with other weapons.
Suicide would also go way down.
When homicide and suicide and warlordism rebounded, the population of democratic nations would see the claim that an armed society is a peaceful one was a myth, and would push hard to go in the direction of the earlier act of God.
A new religion, focused on the blessed day when guns dissappeared, would spearhead this, but many secular people would join in their efforts.
Well, I don’t know for sure that the person who bought guns on my credit card was a felon, but the local cop sure seemed to think so. And I don’t know how exactly the mail order gun store in Oregon checked the record of the person in California who bought it. If they checked my record it would come up clear, but the addresses wouldn’t match. This was a while ago, but methinks there are a few holes in this procedure.