Sorry for your loss!
I had a rooster just die that same way. The prelim from the necropsy was a massive parasite infection in his gizzard. I saw no evidence in his poop for worms or any reason to believe he had parasites.
So if you don't worm your birds, I would start there. Plenty info on this wonderful site to help you with that. He was lethargic, then dead in less that 24 hours. I figured he had an infection of some sort, or a chicken disease. Final results are pending, so there may be more to it, however he was severely underweight and malnourished from the long standing problem (he looked very healthy to me!)
If you did not dispose of her, best thing to do is to have a necropsy done. The state I am in, Virginia, is doing them at no cost, as the USDA wants to be in the loop and is picking up the bill currently. The lab was 1.5 hours away - and the people there were very helpful. You are suppose to refrigerate, not freeze the bird, and I just took him in a plastic bag. Mine got frozen though, but they said to bring him in anyways - as this only interferes with some findings. I'm glad I did.
The experts here have listed lab contacts for the US, and easy to find on this forum.
Again, sorry for your loss - in my short experience - it is always the favorites that die -