Cats can become lame for a variety of reasons, most of which are traumatic. Cats can be hit by cars, injured by rocking recliners or winging doors, be stepped on or suffer falls from windows or other surfaces. All of these potential forms of trauma can cause anything from a broken bone (fracture), sprain or soft tissue injury. Other causes for lameness can be tumors, lacerations or skin problems that impede the ability to walk normally and certain types of cancer.
If you notice that your cat is lame, try to examine the leg. Don't get bitten or scratched in the process. If there is a lesion or the lameness is severe or associated with severe pain, our recommendation is to have your cat evaluated by your veterinarian. If your cat is not painful and is otherwise acting normal, you could wait for an hour to so to see if the lameness persists. Many minor problem will resolve. IF the lameness persists, see your vet.
For more information on lameness, please read Lameness or Limping in Cats.