Can u suggest a leg brace to staighten my leg out?

You need to see doc. Instead of trying to be diagnosed and trtd over the net, you need to be checked by a doc. Inability to straighten leg out, if acute, could imply a fracture. Long-term inability to straighten a leg out could indicate a contracture due to a neurological disorder. You need to go see a doc for proper diagnosis and treatment, as causes are broad. Net is not the place for this. Wrong trmnt can harm.
Seek a medical help. You have to see a doc with musculoskeletal background. Many possibilities that can cause your problem. Appropriate diagnosis leads to a right management.