If you have noticed that your pet rabbit can’t move its back legs, it is likely to be either hind limb paralysis (complete inability to move its legs) or paresis (partial movement). First of all, the best thing that you can do is to take them to a veterinarian who specializes in rabbit care. Some of the common, early symptoms of hind leg paralysis or paresis are weakness in the back legs, unusual hunch in the back end of the back, having issues standing up, and falls over in normal circumstances.
Unfortunately, it is quite common for pet rabbits to have issues with their hind legs. Some of the reasons are injury to the limbs, pelvis, or spine, spondylosis or osteoarthritis, infections, bacteria, or parasites. The earlier that you notice the problem, the better. If you can, you need to get them to the vets as soon as you notice an issue. There are various things that they can do to help. For example, physical therapy, anti-inflammatories, or supplements such as glucosamine.