Sharing Music Between iPad and iPhone

Quite a few years ago, I bought a bunch of music from iTunes and  promptly downloaded it to my PC, and then onto my iPhone and my iPad. Fast-forward a couple years and a couple of computers, and I can no longer find the music that I had saved to my previous computer, however I still have it on my iPad. I can no longer find it in my purchase history on iTunes, but it is also no longer on my iPhone. When I try to AirDrop these onto my iPhone from my iPad, it says “failed”. Also some of the songs that should be on my iPad are no longer on my iPad. However the titles are still there and when they click on them I get the message that these songs are no longer available in my country or region. I really don’t understand. I thought I purchased this music for my own use. What happened to it?