k, ignore 8corewhore, he had the right answer to the wrong question lol
THIS IS WHAT YOU DO (since it happened to me for like 4 bands)
right click, edit the artist's name to something else. "Metallica2" for example. voila, it now just regenerated the album art. (the cover is selected by the album with the highest rating)
Then repeat and name it back to Metallica. Fixed.
Wanted to add my thanks to Lvivske here, as this method worked for me on my missing covers as well, after I migrated my collection from a windows computer to a new mac mini.
However for me the cover artwork disappeared on almost the entire first half of my collection, despite the artwork being properly embedded in the audio files (for some bizarre reason I got almost no artwork for artists between A-F, but all my artwork for artists G-Z showed up fine). The method of renaming the artist and then renaming it back was taking too long, since it had to change each track individually. Perhaps because my collection is on a NAS, each change took several seconds - so it would take 30+ seconds to change the artist name for one album, and then another 30+ seconds to change it back. That wasn't something I wanted to do for 100+ albums.
I tried mcstain's method of playing the first track - that did nothing for me. However,
what DID work was doing "Get Info" on the first track of an album. The artwork I had embedded into the file was there. I just hit "OK" without making any changes, and boom, my artwork showed up for the album. So you don't actually have to change the artist name, or in fact make any changes to the metadata at all - in fact you can just hit "cancel" to get out of the Get Info view, and the album artwork still updates!
So then I made a smart playlist of all the "Track 1"s in my library, and then did a get info (command-I / CTRL-I) on each and now all my artwork is back.
NOTE: this only works on tracks that have the artwork embedded in the file - but I've spent many hours over the years meticulously adding metadata and artwork to all my music files, so I was very relieved to find a relatively easy way to restore that artwork.
Lvivske, thanks again for putting me on the right track! Nothing I'd found worked very well before I found your post.