Take the cat to the vet!! A limp could be a symptom of many things. It could be as simple as a sprain which can heal by itself or it could be a dislocation, trouble with the back and/or spine, or another serious problem.
Cats hide their illnesses because, in the wild, weakness shown means other animals may prey on them. By the time a cat shows it's illness or problem, it can be major.
A vet is necessary at this point.