Missing music after Apple Music and software updates

For several years I've been missing music after Apple changed from iTunes to Apple Music and/or I do a software update. I can see the song in my library/playlists but the songs will not play.I also purchased several songs from iTunes and I had to repurchase because it wouldn't play. I purchased a new Mac 2 years ago and it got even worse because again some music did not transfer over from the old not even in the finder. I've lost a valuable, numerous amount of music.This is insane.Please help!

[Re-Titled by Moderator]