Hi there and welcome to BYC!
I'm very sorry for your loss and your experience, it sounds like you did all you could do for her.
So my top suspects are.. possible crop issue.. or other blockage. You say there was no egg felt.. Did you happen to see any droppings from her? Had she started laying already and if so when was her last egg?
What breed and was she vaccinated against Marek's? What is the feeds nutritional values.. protein, calcium? How many other birds, in how much space, recent weather pattern? Any new additions, recent changes? Do they free range where she might have gotten into something
toxic either plant related, or oil/cleaner spill? What kind of water dish and are the vitamins and electrolytes in there full time?
Consider adding your general location to your profile as it may help folks make their most relevant suggestions possible at a glance.
Do you still have the body? You can refrigerate while making contact with your state poultry lab and send it it to get answers, often free or affordable to back yard keepers as a means of protecting our national food supply chain. Links to help with that since I know TSC is in the US.. It cost $25 for 2 birds in California... They can Or should be able to tell you if it was Marek's,
coccidiosis, Leukosis, or other condition..
State poultry labs
People don't realize that fruit and vegetable are generally good enrichment but not of much nutritional value. Chickens like meat including bugs.. they are omnivores and REQUIRE some animal derived amino acids.. our mostly "vegetarian" formulas we feed have these added in synthetically.. and so a vegetarian chicken would have never seen the ground or light of day because they explore the world around by tasting it sometimes. I think this vegetarian feed thing is in response to mad cow disease and folks don't wan't chicken eating chicken.. which they are crazy for.
I see that you have buried her already.. whether or not the rest of your birds will face it will depend on what it was. If it was something contagious then they've already been exposed.
I still think you might benefit from some Q & A.. for example.. if they are getting vitamins/electrolytes in their water full time.. depending on what they are.. it could very well be overdosed, though I'm under the impression water soluble vitamins cannot that may not pertain to mineral aka electrolytes.. I am NOT an expert.. the info I have say NO supplement should be given more than 10 days in a row unless under veterinary advice.
Hope the rest continue to thrive!