Problem with a song

So August 2018 I purchased a song "One Kiss" by Calvin Harris on iTunes via phone. Everything worked fine, I could normally listen to any time I wanted but for a past month whenever I wanted to listen to it (on a phone, it worked fine on mac) a window on a screen suggested on trying Apple music. The only way I could listen to the song was to go to iTunes store and play it there, the song would normally work even with my lyrics but if I changed it to another song I wouldn't be able to go back. So I tried deleting it and re-downloading it from my account but the song isn't there. It's like I've never purchased it, although I have invoice from Apple. Is there a way to fix it? I really want the song back. Thank you.