Assuming your cat is healthy, often the reason they stop using the litter box is she doesn’t like the restroom accommodations your have provided. Cats have a reputation for being picky and there are quite a few things you can change that might make them happier.
If your litter box is dirty for example, your cat may just decide that he doesn’t want to walk through his own poo to go. A covered litter box may also pose a problem. What we see as a privacy screen creates a feeling of claustrophobia for some cats, not to mention it traps the odors making it even less pleasant. It’s also possible that your litter box isn’t big enough; it should be 1 ½ the length of your cat.
And then there is the litter question. Is there enough litter in the box? Three inches is considered adequate, but make sure you are replacing the it as you scoop it out. Also, are you using a litter that your cat likes? There are many different litter options on the market, you might need to try a few to figure out what your cat finds acceptable.
Visit a local pet store such as Thomaston Feed for additional information on your cat litter options.
Your cat could also be put off by the location of the litter box, high traffic areas, or being near loud appliances might make a nervous cat uncomfortable. Your cat will just go finder a safer, more private place to pee.
And one final, and often over looked point, do you have enough litter boxes? A general rule of thumb is a box for each cat in the household, plus one.
If your cat is healthy, and has started going outside of the box, work through the suggestions and see if you can figure out what is making her go rogue. Just like you, they would prefer a clean, private, safe environment to do their business.