It may be due to compression of the radial nerve roots(called as saturday night palsy).Due to prolonged immobilization in the same direction (for example you just hang your arm by the side of you on the chair for prolonged periods
,then you feel numbness).It can be due to any disc prolapse also,so you may need to consult a neurologist
and get a nerve conduction study
to know which nerve is affected and MRI of cervical spine
to rule out disc extrusions.