My Music Won't Appear on my OnePlus One
An Android Central thread discusses this issue and has several things to try:


Force stop Google Play Music and clear its data before reopening it. This may lose some data such as playlists but hopefully not the music itself.
Uninstall and reinstall all the updates to Google Play Music.
Ensure that the folder that the music is stored in or any parent folders don't have .nomedia files in them. You may need to enable hidden files in your file manager to see these files.


Unfortunately, some of these solutions seem temporary but it may be worth a try.