A sudden change in a cat's urinary habits are most often caused by a urinary tract infection. Other causes you must also entertain is that poor litter conditions. Try replacing all the litter. and cleaning the box. Sometimes the issue isn't with the cleanliness of the box, but rather the location. The box may be in an undesirable location for the cat, such as in a high traffic area. The box may need to be moved. Something else I have recommended in the past for friends who have had cats who have had litter issues is to place a second litter box in the house. This will give the cat a choice of where to go and an second option if they find their first choice undesirable for whatever reason. The final issue may be that the cat feels threatened by something and feels the need to mark and defend its territory. Did you add a new animal to the cat's home? A new person? Is there a stray cat in your yard that may be stressing your cat?