I’m sorry that you lost her. The best way to know what might have been wrong is to have your state vet lab do a necropsy. The second best way is to do one at home, but even then there may not be anything obvious. Look at the major organs and take pictures of the liver, oviduct, intestines, crop and gizzard contents, or anything out of the ordinary. Sometimes we may see yellow ascites fluid in the abdomen, or large amounts of fat, or blood clots. Here is a link for contacting the state vet lab in your state:
https://www.metzerfarms.com/poultry...7IOlHOhP-eD8qMtZ70RNq6BMO9kVUn3x6so7q0Z_JgEr8
This is a good video on doing a necropsy and organs are labeled:
https://www.metzerfarms.com/poultry...7IOlHOhP-eD8qMtZ70RNq6BMO9kVUn3x6so7q0Z_JgEr8
This is a good video on doing a necropsy and organs are labeled: