Cats stop using the litter box for a variety of reasons.
Number 1 is a urinary tract infection and/or painful crystals developing.
Other reasons: smelly cat litter (perfumed cat litter is for us NOT the cat), covered litter boxes....some cats are simply afraid to go in or are disgusted with the small, the litter box is too hidden or too exposed, another animal is scaring them when they are using the box etc.
If this is the first time this cat has done this she well deserves a second chance.
Caretaker of 2 dobermans,, 1 schnauzer, 5 cats, 3 goats, 1 parrot, 1 bearded dragon lizard, 11 chinchillas.
Rest in peace, Cricket, Casey, Bella, Yank, Chloe and Bones.