There are a number of possible reasons why your cat may have stopped using the litter box. Here I suggest some possible reasons that might be helpful to review.
The litter box container has built up bacteria and residue that’s hard for you to detect
You may not be able to detect built-up bacteria but cats have a superior sense of smell. When your cat scratches in the box this may stir-up the odor from bacteria that exists on the inside of the cat box container. For this reason, the plastic bottom liner area and the sides need to be thoroughly cleaned. It’s recommended to empty-out all of the cat litter, discard it, and then scrub the entire cat box (interior and exterior) using a powerful household cleaner. Let the litter box dry completely and then refill the box with fresh cat litter.
The floor and surrounding area has odors that are hard to detect
Make sure the floors are scrubbed and mopped with a strong household cleaner to remove any bacteria odor or residue that your cat may detect.
The cat litter is a new brand
Sometimes switching cat litter takes time for a cat to adjust to. It may seem like a trivial thing but for many cats, this is a routine breaker. There’s nothing more you can really do other than encouraging your cat to use the box. Eventually they tend to catch on and will deal with the new brand. If the problem persists then consider using a second cat box. Use one box with the old litter but keep the box with the new litter near their food source or in an area that they frequent the most. This sometimes works.
There was a break in the cat’s routine
Cats adapt pretty fast to changes in routine depending on their personality. Generally, a little time is all that’s needed. If you have just recently moved into a new home it will take some time for your cat to adjust. A good idea is to cordon-off a smaller area in the house in close proximity to the cat box and keep your cat there for a period of time. Their food and water should also be placed in the same vicinity. Plan your strategy so this area ends up being the permanent place where their cat box will be located. After some time has passed and your cat appears calm, open the area to allow them to gradually explore the rest of the house. This method will allow your cat to explore new territory in small increments. Most cats respond well to this incremental method. Another common routine breaker for cats are when there are strangers in the house. You should try and introduce your cat to your visitors to get them used to new faces. Sometimes having new people in the house is enough to make your cat avoid using the box altogether. If this happens consider cordoning-off an area for your cat where it can access the cat box without being disturbed by new people.
The cat litter box was recently moved
Encouragement can go a long way. Remember that the close vicinity of food and water to the cat box is important to consider for your cat to discover the new location of the cat box. They will find the food no matter where you put it. So use that as a helpful lure to get them to discover their litter box.
There’s a scary thing near the litter box
We may never truly understand the complexities of feline behavior. They often play by their own set of cat rules that seem to defy reason. The survival instincts in our garden variety house cat is highly developed in its own unique way and they may be hypersensitive to things that may seem absolutely absurd. A scary thing to a cat could range from a rubber band on the floor, to a sweater folded over the back of a chair. These things may suddenly and unexpectedly trigger anxiety in a cat. Never dismiss possibilities like this. Also, if you are using a covered litter box it sometimes helps to remove the cover temporarily.
Cat is unhappy because she lacks attention
Attention may be part of a normal routine your cat is expecting. This is common sense but I have listed it here anyway. Is he getting enough of your love and attention? Do you have any reason to believe your can is unhappy? If there are other stress factors present then you might need to compensate by providing more attention.
Cat has a medical problem that needs attention
Sometimes cats will quit using the litter box when they have a urinary tract infection. It can help to move the litter box to a new location or to add a second litter pan in another room. Urinary tract infections often occur with kidney failure. It can also occur with healthy kidneys. The infection can be very painful when urinating so your cat may cry out. Male cats can develop a blocked urinary tract due to crystals in the urine. This is very serious and needs to be treated by a vet as soon as possible. In any case, it’s never a bad idea to consult a veterinarian if you suspect your cat isn’t using the cat litter box due to health reasons.