W124 - car won't enage in reverse

[mbnz]

My car, 1990 300E, today for the first time, won't go in reverse. I was about 50 miles away from home, and fortunately was able to back up because there was a slight hill to roll back from neutral.

I tried moving the shifter around but it won't go into reserve. It won't engage at all. Note, the back up lights do go on though.

Otherwise, the transmission runs fine, I was able to drive 50 miles to get home and everything seemed fine. The shifting between the gears was smooth.

On a related, 2 weeks ago, I had my front flex disc (transmission side) changed. Could this be related? Is it possible that the place may have damaged something when they had to remove the drive shaft to change the flex disc?

Any thoughts and probable causes?
I hope this isn't an expensive repair.