LK,
I injured mine in a motorcycle crash (over the handlebars into a tumbling roll) in the early 80's. Diagnosis was simple - small town MD had me go thru a few arm movements with him providing light resistance. Stnading, with my elbows down, forearms parallel to the floor with palms facing in, his hands against the back of mine, he told me to push against his hands. My right hand rotated outward easily, while my left had no strength and the attempt at movement caused considerable pain. Prescribed regimen was imobilize for a few days, take Tylenol, and begin to use it as the pain diminished.
Over the years other muscles have compensated (I never had it 'fixed'

- occasionally I will have it go numb when I get a military press to far back, or reach overhead and behind my shoulder. Other than that, I've lifted, played golf, baseball, softball, & volleyball with little problem.
Good luck!