What should I do if my cat's hind legs are weak and it can't walk?
If a cat exhibits weakness and unsteadiness in its hind legs while walking, there are generally several possibilities.
The first possibility is that the cat's hind leg is injured, such as a fracture or a collision. In this case, you can press the cat's hind leg with your hand to see if the cat feels any pain. 
The second possibility is that the cat is calcium deficient, which can cause unsteady gait and a wobbly walk. In this case, calcium supplements are needed. The third possibility is that the cat is malnourished; in this case, you can give the cat some nutritious food.
The fourth possibility is that the cat has a medical condition.
In either case, if your cat exhibits symptoms of weakness or unsteadiness while walking, it is recommended to take it to the vet for a checkup.
