If Kitty suddenly can't walk, it's pretty scary for both of you. While losing control of his legs constitutes a veterinary emergency, whether or not he'll be able to walk again depends on the cause of the problem. Don't panic -- in many cases, he'll be up and running again.
Sometimes diabetic neuropathy, or hind leg weakness, is the first sign of feline diabetes mellitus. Your cat's pancreas no longer produces sufficient insulin, so too much sugar accumulates in his bloodstream. That's the reason the disease is often referred to as "sugar diabetes." While symptoms generally include excessive drinking and urination, neuropathy develops when your cat's muscle tissue weakens and he becomes wobbly and unsteady in the hind end. Fortunately, insulin therapy generally reverses the condition. Kitty will require daily insulin injections for the rest of his life.
Feline Aortic Thromboembolism
If your cat is fine one minute and suddenly dragging his hind legs the next, it's quite possible he's suffering from a blood clot, known as feline aortic thromboembolism. Also referred to as a saddle thrombus, the prognosis isn't so good for this condition. Kitty experiences a lot of pain, and as you're putting him in the carrier for the trip to the vet, you might notice his legs are cold. A saddle thrombus occurs when a piece of a clot in his heart breaks off and ends up in the "saddle," around the pelvic area where the arteries supplying blood to the back legs are located. Your cat has about a 50-50 chance of surviving the initial episode. If he does regain use of his legs, it will be a long road back, requiring lots of care.
Cats suffering from epilepsy experience seizures, which not only cause them to temporarily lose control of their legs, but also display other symptoms. These include excessive salivation and loss of urinary and bowel control. Cats might just go through one episode -- and once is more than enough -- or continually have seizures. Even if your cat appears to recover after a brief seizure, take him to the vet for an examination. Your vet can prescribe medication, generally phenobarbital, to help prevent future seizures.
If your cat goes outdoors, he's at risk of getting hit by a car or getting attacked by a dog. These incidents, and other sorts of trauma, could cause damage to his legs and spine. Your vet can take X-rays to determine what happened to Kitty and how he can be treated.
If your cat doesn't regain use of his legs, remaining paralyzed, he'll require special care. He probably won't be able to control his bladder and bowels. Ask your vet about using feline diapers on your cat. It's possible you'll have to manually express his bladder a few times daily. Your cat might have only a limited ability to groom himself, so you'll need to take on that chore for him too.