I believe this is related to the feature on some 5 series mirrors when engaging the shifter in REVERSE, the passenger mirror ONLY travels in a downward direction so driver can see low objects/sidewalks etc while backing up.
Since the mirror got stuck in downward position it is either the motor itself or the switch controlling it by the shifter "R" position. Did you try to wiggle the shifter between the "D" and "R"? D brings it back to its original position; R=downward.
While doing so do you hear the litttle motor in the mirror?