She should see a vet. It is possible for a cat to associate the litter box with pain if they have some sort of infection. Pam Johnson-Bennett, author of "Think Like a Cat" says that if a cat feels pain when they try to pee in the box, they may seek out other areas, hoping that they won't feel pain there.
If you get her checked and rule out medical problems, then you can focus on the behavioral side of it. You might also want to get some Nature's Miracle to remove the odors. Traditional chemical products usually leave enough of the odor so the cat can still smell it, and she will keep going back there.
Also consider whether there have been any changes in the household recently. Even something as simple as rearranging furniture can stress some cats out. Good luck.