There are a few reasons why a cat might lose its hearing (or vision). Accidents can be responsible, as can disease, birth defects, reactions to medications or a lack of oxygen during birth. White cats with blue highs are very likely to become deaf, and white cats with yellow eyes or one blue eye have a higher risk than other cats (but not as high as white cats with two blue eyes).
If your cat is losing its hearing, your veterinarian can assess the situation to see if it's treatable. For example, if the deafness is being caused by an obstructed ear canal due to wax, ear mite debris or a tumor, the problem can probably be rectified. If the problem can't be fixed, you have to learn to adjust to your cat's disability. Deaf cats rely on their sight and sense of touch in order to get by. They see moving objects better than they see still ones, so they might try to make objects move on their own. If you have breakable items within a deaf cat's reach, you may want to rethink their placement. The shatter of a breaking vase doesn't scare a deaf cat, so it won't stop him from pushing more glass objects to the floor in the future. If you want to help stimulate your cat, make things move for them.
Another adjustment you have to make when caring for a deaf cat is that you have to remember you cat won't be able to hear you calling him or reprimanding him. However, your deaf cat can still feel the vibrations of a foot stamp or a knock on the surfaces he's standing on. Keep in mind that if your cat has gone deaf, it's unsafe to let him outdoors on his own.