Help for future reference...

Yesterday we noticed one of our very large buff orpington hens stayed in the run while all of the others came out and began foraging. She was standing and fluffed up and was very lethargic. We watched her for a few hours and isolated her in a crate. We checked her for egg bound and found nothing. She went down hill quick and would not eat or drink and we began giving her electrolytes and she would not even stand up but would lay on her side. We woke up this morning to find she had died and had regurgitated her crop. What could have caused this she was 8 months old. Any ideas would help in case any other birds show these signs. We also completely cleaned coup and run and all other birds are not showing these signs. Thanks