If your cat stops using the litter box, it is important to first rule out any potential medical issues by taking your cat to the vet. Once medical issues are ruled out, you can try adjusting the litter box environment, such as using a different type of litter, cleaning the box more frequently, or providing multiple litter boxes in different locations. It is also important to ensure that the litter box is placed in a quiet and easily accessible area for your cat. Additionally, addressing any potential stressors or changes in your cat's environment can help encourage them to use the litter box again.