Cats can be fickle creatures. One moment they’re perfectly happy and the next they’re urinating anywhere but inside the litter box. Despite having clear access to a clean litter box, there are several reasons why a cat might not use one.
First, cats may be upset if their life routine changes such as the addition of another person in the house or another pet. Second, cats might have urinary problems that could cause them pain. If they urinate and feel pain, they may associate that pain with the litter box.
If your cat suddenly stops using a litter box, look for a reason why. You might never find one, but cats don’t do things without a reason so keep looking and you may find the answer right before your eyes.