If only one light is out, and only the low beam, it is most likely the bulb.
Fuses are cheaper than bulbs, so it can’t hurt to start there. Most fuses are designed to be visually inspected. Open the car’s fuse box, find the fuse(s) for the headlights, and see if it is blown. If you aren’t sure, then replace it. Like I said, fuses are cheap. And you should always have some spares.
There are some devices that function like fuses but aren’t in the fuse box. These are usually harder to replace, so hope it isn’t one of those.
Might be a bad connection. Wiggle the plug where the wires attach to the bulb. Unplug it and plug it back in. If that fixed the problem, yay. If it didn’t (and it probably won’t), then either it is a bad connection somewhere else (which will be hard to find and therefore bad), or it is the bulb.
If it is a cheap bulb, buy a replacement. If it is expensive, test it to see if the circuit is broken inside the bulb.
So you’ve got a brand new fuse and a brand new bulb and the light still doesn’t work? You have Bigger Problems™. Something is wrong somewhere in the wiring.
Armed with a circuit tester, a lot of patience, and hopefully a wiring diagram that tells you which color wires do what (which are a part of any repair manual for a car made before 1980, but not after for some reason), you can track that sucker down.
OR you could pay somebody to do that for you.
At this point we are into seriously low-probability events. Most likely, it was the bulb.
Fun story: In 1989 I bought a 1980 Mazda GLC. Shortly after I bought it, the headlights stopped working completely.
I took it to a shop that specialized in automotive electrical repair and asked him to answer one simple question: is there power at the switch? He said yes.
This suggested it was a “fusable link” in the wires to the headlights. Which I could not find (it would appear to be a thicker section of the wire, and I just couldn’t find anything like that).
In my growing frustration, I decided to verify the test results myself in a very primitive way: I went to the base of the steering column and found the wire that supplied power to the headlight switch, then I found the wire that supplied power to the parking light switch. I cut both, and connected the headlight switch to the parking light power.
Headlights worked fine.
Then, this being me, came the kludge. I connected both the headlight switch and the parking light switch to the power from the parking light fuse, wrapped the connection with electrical tape, and pronounced it “fixed”.
My understanding is that every time I turned the headlights on it should have blown that fuse (the headlights had previously been on their own circuit with a higher amp fuse), and I still don’t know why it didn’t, but it didn’t. The headlights worked fine for the few more months before I sold that car to a junk dealer.
But I never forgot that I had paid an electrician $30 to narrow down my search for the problem (before or after the switch), and I got the wrong answer.