Born Again 05 is correct.
If you haven't changed anything regarding your cat's litter box such as the type of litter, the location and privacy, it is time to consider a medical issue.
Your older cat needs to be thoroughly checked by a veterinarian to be sure there is no pathological cause of this inappropriate elimination.
If, after your kitty is cleared by the doctor, you might want to reevaluate the box. Can he/she get into it easily? Is the litter comfortable for his or her paws? Is the box in a quiet location? Has anything else in your household changed?
Cats are creatures of habit. Any change to kitty's routine could cause her to not use her box.