Yesterday all my music disappeared overnight. Both on my phone and my Apple Watch. I went the whole day trying to figure out why. Even my purchased songs no longer showed in the music app. No songs or playlists. Absolutely nothing. I tried to disable iCloud music library, Apple Music, sign out and back in. I managed to get my purchased music to show up but as fast as it showed up they all disappeared again. Finally I fixed it.
I Opened my MacBook and quickly disabled wifi. Just in case it was an error with Apple Music I didn't want iTunes to sync and wipe my library. So I checked iTunes saw my nearly current library and connected back to wifi. My library on my Mac updated to display the songs I added the day before and shortly after my music on my iPhone showed up. My playlists and all. The only downside is that I had to redownload all my music back to my phone and watch since for whatever reason all my previously downloaded music was showing up as unavailble offline.
Hope this helps!