imported CDs missing from iTunes on Mac

As of this morning most of my imported CDs are missing from my iTunes library. The music is still in the iTunes library folder and iTunes still has the right folder as the place to look for music, but most of the albums aren't showing up in iTunes. I'm using iTunes 12.8.3.1 on a MacBook Pro running High Sierra (I know. I'll get a newer one someday). It's been working fine for ages so it was a bit of a shock to see so few albums listed. It doesn't seem to have anything to do with whether they've been played recently as some of the items listed have never been played and some of those missing are played regularly. I have View>All Music checked. Perhaps there is a way to get iTunes to re-read the folder to resynchronize it with what is there. Some of the music listed was purchased from the iTunes store but most (like most of the music missing) was imported from CDs.

I do have an Apple Music subscription, but it isn't new.

I tried running Disk Utility on the iTunes volume (it showed no problems) and unmounting and remounting it (no improvement).

Thanks