C5 motor nerve impingement or Suprascapular Nerve Entrapment The most common places for this to occur being at the nerve root exiting the spine at C5 C6. There are other locations in your upper back and shoulder where impingement can occur . Either way it SUCKS!
You could try a chiro, neck traction etc. In my experience with this which is way more then I care to remember, the best thing you can do is determne where the nerve is most likely impinged. Ice the area several times a day until completely numb. Avoid anything that might cause infalamation in your upper back/neck. It takes time for the nerver to recover once you start experiencing weakness. So get the inflamation down asap and give the nerver time to recover.
best of luck to you