If your cat suddenly stops using its litterbox, your veterinarian may be able to help you get to the bottom of the problem.
Have your veterinarian take X-rays of your cat's lower back to check for arthritis. Many older cat patients suffer from this problem, making it painful for them to climb in and out of the litterbox. Also, have a urinalysis performed to rule out diabetes or a urinary tract infection. If the veterinarian finds that your cat is constipated, that problem needs to be addressed. Work closely with your veterinarian to get the necessary steps taken for diagnostics.