wow. all my music is gone, aside from songs purchased off iTunes. all my playlists as well. i assume i will get them back after i sync with iTunes at home, but this is pretty lame. i did not sign up for Apple Music and i am not on Match. i haven't read this whole thread, but i certainly wasn't aware updating to 8.4 would delete my entire library!
[edit] the space my music took up is still used, but under General > About > Songs = 0.
I updated thru iTunes 12.1.. and have no issues on my existing music. I did not select to accept Apple Music and other than a new layout, all songs, playlists are there.