He probably had been sick for awhile before you noticed. Animals generally conceal any illnesses or injuries as long as possible. In the wild, showing signs of vulnerability is a way to end up getting eaten. So animals often don’t look sick until they’re on the verge of dying.
This, particularly prey animals.
But, since you want specific conditions, there are two main geriatric conditions which can cause death in hamsters. Since this was a hamster living alone, I’m not including possibly infectious diseases which could be more common in colonies. The conditions are amyloidosis and atrial thrombosis (blood clots in the heart). The later can be also caused by the first one, as kidney failure due to amyloidosis can lead to clotting problems which can lead to getting a blood clot in the heart.
For more information about these diseases, this is a good link: Hamster handout at Veterinary Partner.
Thanks so much guys.
I really needed closure over Hammie.
In my experience I have found that most mammals that die were alive the day before.
Yeah, it’s the hammer part that’s probelmatic with hamsters.
It’s never lupus!
Yeah, my hammie died of a stroke awhile ago. Usually it’s not too much pain.
Obligatory mention of zombie hamsters. Interesting because apparently you can freeze them and then re-animate them. (Don’t try this.)
I’ll just say that goldfish are also excellent at dying. 100 gallon tank with full filtering, stable pH and chemical levels, only 3 fish in the tank, and they’ll still die. They’re like the sheep of domestic pets. Able to die in new and inventive ways for no particular reason. :mad:
Could be, though. If your hamster gets loose out in a field and gets eaten by a wolf, it might be lupus.
Was there a tiny noose and a puddle of water in the cage?
Hamsters really are “tame” and not really what we think of as “domesticated”. Wild animals generally do not show outward signs of weakness because in the wild this gets them eaten. When it is time for them to go the end is usually swift.
Oh, I don’t think a swift could carry off a hamster, could it?
African or European?
Taylor Swift kills hamsters?
Ha! A trick question, eh? The unladen bird of Monty Python was a swallow. It might well have been very swift, but I don’t think the matter of its speed was properly resolved.
No, silly! That was Freddie Starr. 
(Or not, really).
Here’s a hamstery question (or two)
1 - Would it be VERY odd of me, adult woman of many years older than the usual hamster-keeping age, to get a hamster or two as pet (s)?
2 - Were I to adopt a hamster, would it be massively cruel, or might it be all right, to leave it all alone for the occasional weekend? With plenty of food and water, obviously.
Do any of you realize you’re answering a thread that’s 3 years old?
Which goes to show that some SDMB threads have a considerably longer life span than Syrian hamsters!