There are so many possibilities. Can you elaborate on the circumstances and maybe describe the bunny's lifestyle a bit? (For instance, does she have access to a yard where she may have eaten something toxic? Are there very young children in the house who may not understand how to properly hold a rabbit and perhaps inadvertently injured her?) Without some particulars, it would be impossible to guess what killed her.
What kind of rabbit? What was the bunny fed? What was the bunny given as treats and for toys?
The chief reason I have heard for premature rabbit mortality is overheating.
The next is fur block. Rabbits clean themselves like cats, but lack the ability to vomit, meaning they either pass all the fur they swallow, or they die. We feed papaya that we dry ourselves to our pet Flemish Giant rabbit, as it is rich in an enzyme that helps them to pass fur blockages.
Rabbits need to eat constantly, I think every three hours at least. If anything stops them from eating, their systems can shut down and they die.
I was told this was the prime reason for rabbits to die after operations of any kind, especially involving anaesthesia.
I hope you can track down the cause. Good luck, and keep us posted.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein