What is on her vent—clear drainage, diarrhea, egg white, etc? How does her crop feel—empty and flat, some food in it, firm doughy, or puffy/squishy? How many birds do you have and what breeds? Do they have access to crushed oyster shell, and get outside to roam? Have you seen any signs of vent pecking or bullying? Could they have gotten into any moldy feed? Any dampness or sweating in a feed bag or a few drops of rain could cause this overnight. Are they eating a balanced chicken feed, with very limited or no treats? Your state vet lab could do a thorough necropsy, and others here may advise you. Sorry for your loss. Here are some links about necropsy:
https://hamilton.tennessee.edu/necropsy-services-in-tennessee/
https://www.metzerfarms.com/poultry...TtD4cHQI0lIhuXtIT2Gbd6oy5I5TsEY2sEuMKjxs4co-Q just looks clea