We can’t diagnose any medical problems, you will need to take her to a vet.
If a vet concludes that there is no medical issue, then it can sometimes be that they are bored of the type of hay you regularly give. They need variety to keep interested. Hay needs to be the biggest part of their diet so it is important that you get her eating it again.
However as i said though, a vet will need to take a look at her to rule out any medical issues first.