Someone else pointed out that there are many ways to work around this issue, and it's true.
I tried to log out / into iTunes to no avail.
I tried turning iTunes Match off / back on without any success.
I looked in my iTunes Music folder for any potential duplicates, but there weren't any.
So here's what worked for me.
First, I quit iTunes.
- go to the iTunes menu, then choose Quit iTunes at the bottom
Next, I opened a new Finder window.
- click once on the desktop, go to the File menu and choose New Finder Window. (keyboard shortcut Cmd-N)
I went into my Music folder, then from there into iTunes.
- double click on Music, then double click on iTunes
I selected these files and put them in the Trash (but did not empty the Trash):
- iTunes Music Library.xml
- iTunes Library.itl
- iTunes Library Genius.itdb
- iTunes Library Extras.itdb
- click on one of those, then hold down the Command key, and while continuing to hold it, click on the other files. You should see them highlight and remain highlighted. Then when you have them all, either drag them to the Trash in your dock or again hold down Command and press the delete key to move them all at once.
Don't empty the trash! If this doesn't work, you may want to move them back where you found them and head for the Genius bar at your friendly neighborhood Apple Store.
But to see if it works, open iTunes again.
(iTunes immediately creates fresh copies of the files you moved to your trash, and that's what we want)
When I did it, it started finding my music right away, but if it doesn't:
- in iTunes, go to the Store menu and press Turn on iTunes Match
Hopefully, all your songs will return to their playable status.
If that doesn't work, and you want to go back to where you were when we started, do this:
1) Quit iTunes again (same way you did above)
2) Go back to the iTunes folder and find those same four files that iTunes just made fresh
3) rename all of them (maybe just put an 'x' at the beginning - just so you know they're the new ones)
4) move them to the trash
5) open the trash, find the original ones you moved earlier and put them all back in the iTunes folder
I bet if I were more scientific, I could narrow it down to one of those four files that's the problem.
I hope this helps!