All I can think of is retraining. Restrict her to a small area - start with a crate if you have one, a bathroom if you don't. A small area and a litter box, food bowl and water dish.

After 3 or 4 days of this, allow her a larger area, but still not the full house. See if she maintains using the box.

Oh, and take a urine sample in for testing (most vets allow just the sample if they have seen the cat recently) for a urinary tract infection.

Good luck!