Purchased songs have to be repurchased/gone from library

I wanted to listen to a song today I KNOW I bought several years ago. I know I did because I used the song from my library with an iMovie video. Today I wanted to listen to the song and it has no record that I purchased it and wants me to purchase it again. This has been happening a lot and now today confirms it. Why are our music purchases being deleted? How do we make it stop. I always purchase my music because I value artists and want to pay them. I have created a rich and diverse library of music I love over the years with iTunes. Why do I have to buy a song again? What’s worse is I can’t tell all the old songs that have been deleted though an old iPod has helped confirm some of the deletions. This is decimating what was a great library of music. It’s not a case of I’m in the wrong app. I went to iTunes. I checked my purchases. It’s gone along with others. What is going on?