Why would a cat stop using the litter box?
When a cat stops using the litter box, there's always a reason. Your first impulse may be to shout at the cat, but that will just frighten and confuse him. Cats are not vengeful, and he is not going outside his box to get back at you for pushing him off the sofa. Instead, look at what may have triggered this change, which you yourself may have inadvertently caused.
Because cats are habitually clean, Frisky may be upset by a litter box that isn't changed often enough to suit him, even if you think every other day is OK. Would you use a bathroom with an unflushed toilet? Changes in household routine may also be cause to forgo the box. If you've added a new cat, the resident feline may dislike using the same litter box as the newcomer. He may also not appreciate that you've replaced his regular litter with a new-fangled one. Cats are keenly aware of the scent and feel of their litter, so if you've changed brands or types to take advantage of a sale, your cat may not approve of the savings the way you do. A move to a new home or a student back from college for the summer can also upset him.
Using the carpet rather than the box may also be a sign of disease. Cats suffering from urinary tract infections may associate painful attempts at urination with the litter box, and avoid it. A cat who's thirstier due to kidney, liver or thyroid conditions needs to urinate more frequently, and may not get to his box on time. Similarly, feline diarrhea or constipation may find him desperate to go without caring where.