I'm sorry for your loss.
You might want to look around his cage very carefully to see if he chewed/ate something he shouldn't have. I lost a degu to an impacted or lacerated bowel after he chewed some of the wire (!!) cage he lived in. By the time I realized there was something wrong, it was too late to do anything for him.
We've already removed everything from the cage and inspected it all. Neither of us see anything amiss. It is basically a really big rat cage, with a very deep plastic base. All the toys - the bridge, the log with the seeds in it, the crawl-in-it-and-chew-on-it log, were all new and, except for getting into the seeds and alfalfa stuffed in the one log (which seems to have had quite a bit eaten), nothing seems to have been gnawed on. The few other remaining items in there were Thor's, but they had been carefully sanitized with hot soapy water, detol wipes, bleach, and then washed with detergent and rinsed again for good measure. Anything that could not be sanitized was thrown away and new stuff bought. I even threw away all the treats that we'd bought for Thor, and got new ones.
Except the nibble rings. We got those a couple of weeks before Thor died, because Pets at Home didn't have his regular brand.
Oh. no.
I hope that's just a coincidence there. Thor was quite old. But when I found him he was in the same kind of position - as if he'd been walking and just fallen over dead. Not curled up in his bed, which has been the usual for us for a very old hamster. Surely if there was something wrong with the nibble rings there would be other people having the same problem? I don't remember the brand, got it at Pets at Home and emptied it into the glass treat jar and didn't keep the package. The Daughter will be able to recognize the brand if she sees it. I haven't tossed those out yet. I wonder if I should send a sample somewhere to have them tested?
I just took Fuzz Aldrin back to Pets At Home and got a refund. They had no idea, either. Said nobody else had brought a hamster back dead lately.