Problem(s) Encountered:

I lost the guide star abruptly on my very last standard of the night, and attributed it to the very bright sky or maybe the clouds that had been approaching on the satellite maps.  Then,
when I went to stow, the telescope went to the correct dec but would not move in hour angle, even with the paddle.  Fortunately it's stuck just a bit east of the meridian so it was easy to 
close the dark hatch and fill the dewar.  I don't know whether the guide star loss was due to a failure of the RA motion, but it seems likely.  

Solution:

Secondary drive wheel (black cog at bottom of drive train) was seizing up on a crossbeam support.  Pulled the whole drive assembly off/apart and found that the bearing race for that cog had walked out roughly 1/2”.  Upon removal, found that the bolt holding it in place, located on the back end of that assembly, had come completely loose.  Re-set the race, replaced and tightened the bolt and re-assembled the drive train.  Tested first without load, then with load.  All is well with it again.
Interestingly, the motor was extremely hot when I arrived.  Evidently it was trying to move ever since the last command was given.  I found that somewhat surprising.  In the event that we have a motion failure like this again, it would likely be a good idea to flip the “Drive Motors” switch, located on the front panel of the MDC in the computer room, to OFF.