Same with me on iPhone 6. I was listening to music around lunch time and couple of hours later all songs in my music were completely gone! Got no subscription, I have disabled automatic updates and have no iCloud or anything. I usually sync with cable but have my wifi sync enabled. But at the time this happened I was at work and my laptop with my iTunes library was at home.
The other thing I've noticed is that Facebook Messenger somehow reset itself so I had to click through the welcome screens and set it up. But all my messages were there. Not sure if these two were connected or there was some major glitch in my phone that did that to both apps.
Do you have any suggestions or explanation why this happened? Can't have 15-20GB of data just wiped clean. That's not good at all.