Why am i in a wheelchair and yet the doctors can't diagnose my illness?

Difficult question: There are typically only four areas where the problem may be: the brain which includes the psyche, the spinal cord, the peripheral nerves, and the muscles. The last three are easy enough to evaluate with one or more of the following: MRI, CT, spinal tap, nerve conduction studies, blood tests, muscle biopsy. The brain is more difficult. Some problem lie just outside modern diagnostic capabilities.