Inability to use the back legs is very serious. One possibility is that this kitten experienced a traumatic injury--either during the birthing process or shortly after birth--mom may have laid on the kitten causing damage. This could also be a congenital problem--this kittens neurological or skeletal system did not form correctly. This kitten needs to be examined by a veterinarian--they will determine if this is neurological in nature. X-rays may be taken to assess the bony structures of the spine and hips. The prognosis for recovery and general quality of life in this kitten is poor.