Thanks Valerie. After a while I felt up to trying to find the reason and did a necropsy. I think I came up with a couple of possibilities on what the cause of death was.
First things I looked at were the condition of the comb and wattles, and they looked perfectly fine. Eyes looked good, and the skin was in good shape. No signs of external parasites. The crop was still pretty full from yesterday, though, and felt kind of mushy/grainy, which I would guess is normal.
Inside the beak/mouth looked good. It smelled a little sour, but I'm guessing that would be expected from a chicken that had been dead overnight. There was quite a bit of muscle tissue on the breast bone and the rest of the body, and she felt heavy so she wasn't malnourished.
Looking inside the body there was quite a bit of fat around the vent area, but very little internally around the organs, so I think she may have been a little overweight, but not by much.
I found a single roundworm about two inches long inside, the only internal parasite I found. There were a lot of small chunks of safety glass in the gizzard, and most of it had rounded edges. I don't think that was a problem.
Liver and heart looked healthy on the exterior, as did the intestines. There was one egg that was full size but hadn't been coated with calcium, and another soft one about 1/3 that size in the oviduct.
Two things I found were suspicious.
There were some blood clots in the body cavity, near the heart I think. Maybe a sign of infection or heart problem?
And there was a wadded up softshell from a egg that evidently broke inside her at some time, and may have caused an infection. I don't think I cut into the oviduct so I'm not sure how it got into the body cavity.
There has always been one of my chickens that layed softshell eggs every once in a while, and I suspect that she's the one that died. I guess that's what can happen when chickens are bred to lay lots of eggs.
Edited to add:
I just remembered that even though there was food in the crop and in the gizzard (with the rounded chunks of glass), the intestines were completely empty. I don't know if that means anything, but it does seem strange.
Edit #2:
It's possible the blood clots were near the liver, so I read up on Fatty Liver Hemorrhagic Syndrome. The following article says that it can cause sudden death, but the comb is pale from internal blood loss. The comb of my chicken was still bright red and plump, and the liver didn't look yellowish, so I don't think that was the cause.
https://www.merckvetmanual.com/poul...e/fatty-liver-hemorrhagic-syndrome-in-poultry