Urinary problems in cats can be due to a medical problem or a behavior problem. When cats no longer want to use the litter box the first step is always to see your veterinarian. Your veterinarian will want to check a urine sample for a medical problem. The medical problem could be a feline urinary tract infection or due to crystals or stones in the bladder. Some male cats can become blocked and unable to urinate. If your cat is straining in the litter box and not producing urine (can be confused with constipation) call your veterinarian immediately, this is an emergency situation. Behavior problems are diagnosed after ruling out a medical problem. Behavior problems are especially hard to fix, but not impossible, first try to solve why the cat has decided to stop using the litter box.
There are many reasons why cats stop using the litter box, they are very sensitive creatures after all. One reason is due to surface preferences.
Problem: Some cats have a surface they prefer to use the bathroom on.
This is a guideline for one litter box issue. If your cat is not using the litter box talk to your veterinarian first then try some behavior/training solutions. Contacting an animal behaviorist may also be helpful. Don't give up on your cat, training cats can be done it just takes a little time and a lot of patience. Stay in contact with your veterinarian and keep them apprised of the situation, if training alone doesn't help there might be medications that can help. Most medications are meant to be used along with training.
Related Articles: Cat Litter Box Issues, Cat Urine Issues, and Cats With Behavior Problems
References: Kansas Humane Society
Dr. Emily Weiss, Kansas Humane Society Animal Behaviorist