Im very sorry about your pullet and the shock of discovering her dead when she seemed perfectly healthy the day before. In addition to heatstroke and heart attack as mentioned by @TillyPeeps , a common cause of sudden death in chickens is fatty liver disease. (Medical name fatty liver hemorrhagic syndrome.) It kills suddenly and with no warning, and is sadly common in layer hens that eat too many carbs in proportion to their regular feed. Here is an article that explains fatty liver disease.
https://www.merckvetmanual.com/poul...e/fatty-liver-hemorrhagic-syndrome-in-poultry
HHh
Here is a thread where a hen was officially necropsied, and the cause of death was fatty liver disease. Follow the links within the thread because they contain considerable additional information.
https://www.backyardchickens.com/threads/fatty-liver-disease.1578234/
https://www.merckvetmanual.com/poul...e/fatty-liver-hemorrhagic-syndrome-in-poultry
HHh
Here is a thread where a hen was officially necropsied, and the cause of death was fatty liver disease. Follow the links within the thread because they contain considerable additional information.
https://www.backyardchickens.com/threads/fatty-liver-disease.1578234/