I don't have enough info to figure out what happened. I'm reporting this here to see if it happens to anyone else, maybe gather more data.
I imported 3 CDs worth of tracks, the import proceeded until the last track (45 of 45) and then went no further.
I tried to cancel in the import window. It said it canceled (the cancel button greyed out) but nothing happened, no new music in the library.
I tried to exit MC, it said I couldn't because import was still running, did I really want to.
I said yes, but it didn't exit, got the Mac Beachball.
I then forcekilled it.
Restarted, reimported, and it worked.
Music was being imported from a Google Drive folder, so it is very possible that was the source of the problem.
Respond to this thread if you have similar hangs importing.
John