Weezy,
So sorry to hear this, terrible shock it must have been finding her dead in the nest box. Did she die laying an egg do you think or does she commonly sleep in the nest box? One idea for is you might get the book "The Chicken Health Handbook" as on page 164, it lists 14 "Diseases Causing Sudden Death" that affect mature hens, plus a bunch more that affect chickens of other ages. Each disease is described in the back of the book in great detail. Some possible ones are not transmittable, others are. I tried to look up several to see if I could help you but not having the specifics of her condition before and after dying, I'm not having any luck. I reference this book constantly for everything to do with chicken health, from increasing immunity, bio-security, parasites, etc. I got it when my chickens had a respiratory infection and although I could not narrow it down to the exact disease they had, I sure learned a lot from the book and other resources. Respiratory infections are not easy to distinguish from each other and there are a lot of different ones because chickens have delicate respiratory systems. I got my copy used on
Amazon.
Other than this... you could call your county extension office and see if they know where you can get a necropsy done (that's an autopsy on an animal). Some places do it for free, some for a small fee. Mine will do 3 birds for $30, never had it done but it's good to know it's an option in the future, if I am ever unfortunate enough to need it.
I also googled "diseases causing sudden death in chickens" and another thread on backyard chickens popped up at near the top. It suggests a few things you can look into: poisonous plants and Salmonella, plus a few other things. The link is:
https://www.backyardchickens.com/t/42111/sudden-chicken-death/10
In any case, it's probably going to take some detective work to figure it out but worth it, if you can in any way ward off any further harm to your remaining healthy hens. Good luck and let us know what you find, always interested in the outcome of these as we often hear the beginning of the story but rarely, know what happened in the end.
Best of luck to you,
Guppy