It's possible that it just hasn't reseated properly since you knocked it. Try folding it all the way in and then back 'til it clicks into place. That might just do the trick.

If that doesn't help, I've also found that moving the glass around manually can "unstick" a recalcitrant electric mirror.

Dismantling mirror assemblies usually starts with removing the glass. That should just unclip from its mounting, but this can be tricky to achieve without cracking the glass. After that, you should find all the important bits exposed.

NB: generic advice here, not specific to this car.

I wonder if it may be wise to adjust the mirror regularly to keep the mechanism free - in my case it probably hadn't been moved for about 3 years.

Agreed. Having once several years ago had to revive/replace both motorised adjustment assemblies on an Opel, purely because they'd seized through disuse[1], I now make a point of moving the ones on my cars through their full travel every few months. Never had a problem since.

[1] Grease had set. I stripped, cleaned, regreased and reassembled the first one. The second one did not respond to this treatment. I reckon the two little motors had failed from fighting against the drag of the hardened grease.