You’re probably using either iTunes Match, a standalone service, or the Apple Music subscription, which also includes iTunes Match.
iTunes match scans your local library and matches its contents with what’s available on iTunes to get an anywhere, any time, Apple Music (subscription) style listening experience. Apple Music made everything streamable, so largely rendered iTunes Match useless.
So if you used iTunes Match or Apple Music with “Sync Library” turned on, Apple tried to match your local music files with versions in its own catalog. To make them accessible anywhere.
The original files, if you checked “Keep iTunes Media files organized”, will still be available in the iTunes Media folder. If you didn’t check it, they will be available wherever you left them.
The distinction here is that you can no longer listen to those songs using iTunes Match. The iTunes licensing agreements probably expired. But you original music files are still on your computer. You just can’t play them through iTunes if it’s looking for the cloud version.
If you want to listen to those songs on iTunes, just turn off “Sync Library” and add those files again.