Could be that she's had a scare outside? a new cat nearby that she's had a set to with?
Also very important to have her checked out by a vet as she could have cystitis or kidney probs, or diabetes...lots of illnesses could make a cat suddenly pee in a weird place.
Short term I would suggest from experience, that you get a decent sized litter tray and fill it quite generously with naturalish litter. It doesn't have to be forever, but at least you might be able to stop her from doing it in other areas.
Good luck.