It's unfortunate, but if you deal with living animals, you eventually have to deal with dead animals. As Terry said, it happens. Sometimes you can figure out why and sometimes you can't. It may be a birth defect where an internal organ just was not right and finally gave out. It could be an injury or a disease. Sometimes it is a combination of things, like a weak heart then the stress of laying in the heat, or maybe weakened from parasites plus some other stress. It's just hard to say. If she was in the nest like that, maybe she was eggbound?
I had one last year that died while laying in the triple digit heat. She had not been acting quite right anyway, so I think she was weakened by something then the stress of laying in the heat was just too much.
A lot of times it was nothing you did and nothing you could do. I suggest checking them for mites and lice just to make sure those are not weakening them. I trust you know that for roost mites you have to check after dark. I would not immediately run out and worm them unless the others are not thriving or you see some other symptoms.
Are your nest boxes unusually hot? Are they maybe on the sunny side and with poor ventilation so they trap heat? Even in our recent triple digit heat I've seen three hens pile into one nest box to lay together. That has to get warm. I know this is a stretch but I'm trying to think of any possible explanation.
Just observe them and if the others are acting normal, it was just something that happened. From what you described, I think she had an underlying problem and the stress of laying was just too much for her.
