First of all you need to get the cat checked at a vet to make sure nothing is medically wrong. Second of all you need to get your cat spayed. Have you changed the location of the litter box? Have you changed litter. Cats are very sensitive and sometimes changes will cause things like this. If there is no medical problem and you have not changed anything then you will need to retrain her to the litter box.

I am sure someone else can tell you better how to do that as I have never had to do it and don't want to tell you to do something that is not correct.