Cats stop using the cat litter box for a number of reasons. Once you discover the reason why and correct it, you may need to re-train your cat to use her box again. Whatever what you think, you can train a cat! For most cats, it's not hard. They
instinctively want to use a place with sand-like material to do their business.
To re-train your cat back to her good litter box habits, confine her to one room. It's hard to say exactly for how long, but most cats are back in the habit within a week.
Pick a nice sunny room for kitty. Set up a clean cat litter box, food, water, toys, and bedding for her to sleep on. Be sure to put her food and water as far away from her litter box as possible.
Keep kitty in this room while you're gone. Many cat owners purchase kitty DVDs for their cat to watch all day long while they're gone. These DVDs feature birds, squirrels, chipmunks, and other critters that dart around. You can set up the DVD to play in a continuous loop all day long. These videos are also available in VHS tape format.
Let kitty out under your supervision. If she looks like she's heading for her favorite corner to make an illegal dump, get her back to her room immediately, and place her in the cat litter box. If she uses it, praise her.
Generally, cats "get it" very quickly. For one thing, they get lonely being in a room by themselves. For another, it's a cat's
natural inclining to use a litter box. As long as you've corrected the source of her problem with the litter box, she'll go back to using it.
If you can not let your cat out and supervise her, be sure to visit kitty often. Sit in the room with her, and keep her company. Watch the DVD with her!
Confinement is not cruel if it's done properly. If your cat will not use the litter box during her confinement period, then the cat urine odor damage is limited to one room. If after a few days, your cat is still not using her cat litter box, please consult with your vet. There may be a defect issue that you simply can not tell from observation.