It's really hard to say, it could be so many things. Has she been laying through the winter? You can check her over very carefully to see if she has a lice/mite infestation that is making her weak. Is she on a regular deworming schedule that would rule out a heavy infestation? Usually with worms though you see a gradual decline. What do her droppings look like? Check and see if she feels like she's lost weight. Is she making any kind of oddball breathing noises? Wheezing, rattling, gurgling? If her comb is dark that can indicate heart/circulatory problems. Sometimes all you can do is try to keep them hydrated, warm and try to get food into them and see how it goes.