Cats are usually fastidious creatures and prefer to use a clean and appropriate area for their bathroom needs. When a cat suddenly stops using the litter box, it can be concerning for their owners. There are several reasons why a cat may stop using their litter box, and it is important to identify the underlying cause to ensure their health and well-being.

1. Medical Issues: Urinary tract infections (UTIs), bladder stones, and other medical conditions can cause discomfort while urinating, which can lead to litter box avoidance. Cats may associate the pain they feel with the litter box, and therefore avoid using it. Other medical issues like arthritis can also make it difficult for a cat to enter and exit the litter box, causing them to seek an alternate location to relieve themselves.
2. Litter Box Problems: Litter boxes that are not cleaned regularly, or those with a strong odor, can be unappealing to cats. Additionally, some cats may not like the type of litter being used or the size or location of the litter box. It is important to provide an adequate number of litter boxes in the household, as some cats may require multiple boxes to feel comfortable using them.
3. Changes in the Environment: Cats are creatures of habit and may feel anxious or stressed when their routine is disrupted. Changes in the household, such as the introduction of a new pet or a new baby, can be stressful for cats and cause them to stop using the litter box. Similarly, changes in the location of the litter box or a change in the type of litter used can also cause anxiety for some cats.
4. Behavioral Issues: Behavioral issues like anxiety or marking can also cause a cat to stop using the litter box. Cats that are anxious or stressed may choose to mark their territory by urinating or defecating outside the litter box. This behavior can also be seen in cats that are not spayed or neutered.

If a cat suddenly stops using the litter box, it is important to take them to the vet to rule out any underlying medical conditions. Once medical conditions have been ruled out, it is important to evaluate the litter box and the environment to identify any issues that may be causing the behavior. Providing a clean and appropriate area for the cat to use, as well as reducing stress and anxiety in the household, can help encourage the cat to resume using the litter box. In some cases, consulting with a veterinarian or animal behaviorist may be necessary to address the issue.