Thanks for writing to me.
Most common cause of inability to raise the am after a fall is a fracture. If you have already had an xray done, and the doctor ruled out a fracture, then the next common cause is rotator cuff tear. An MRI of the shoulder can reveal this really.
Only if these two conditions are doled out, will I think of a neurological cause for inability to lift the arm.
If you have any further queries, please wrote to : firstname.lastname@example.org