I purchased music, and then lost some pieces in the library.

I used a $50 iTunes Gift Card to purchase classical music back in 2011, but then, several years later, one piece is missing from the library. I don't know how, and it is probably due to the Apple Watch automatically bringing me to the music app each time it unlocks, and it automatically changes the volume, and due to my hand movements whilst in bed or exercising, I might have deleted it and it is no longer in my account. How can I get it back?