iTunes songs inaccessible after macOS Tahoe 26.0.1 update

I have many songs I've recorded and uploaded to iTunes. Now I can't access most of those songs. Why and how do I get them back?

[Re-Titled by Moderator]

Original Title: I just updated my software to Tahoe 26.0.1. I am a musician and record my sessions with other players and had them in itunes. The most precious songs are ones I've recorded with my brother and sister many years ago. Now they won't play.