Problem
Inside your music library is all your music that you’ve got stored as MP3s. These are all imported into Groove Music so that you can listen to them. After a while your music is disappearing from your Music Library, but you can still access it in Groove Music.
Solution
Groove Music has moved some of your music to C:\Users\Public\Public Music. How I found this out is to right click on a music file that is no longer in your Music library, but is in Groove music, and then click open file location. It might put it in difference places depending on your setup.