I went through this a month ago. If "resetting" the mirror does not work (manual moving it 90 degrees), your only option is to replace the mirror motor. I believe the part is like $485?
The sad part is the dealer said it was only a gear that had stripped, but they replaced the whole thing. Things don't get fixed anymore, they get replaced.
It's not like you could buy an individual gear for a mirror in a car anyways.
*covered under CPO*
Try searching how to reset the mirror though. You'll find at least 3 or 4 threads on e90post about it.