iCloud automatically logged out and other nuisances followed

Hello,

I woke up this morning and my iPhone 6 Plus (running iOS 9) prompted me to sign in with iCloud (apparently it was logged out for no reason). I tried logging in but it didn't work. I also found out that most of my other social accounts like Facebook, KakaoTalk and Line were also logged out, including iMessage through which my mother has been trying to reach me but the texts came as international SMS (I live abroad). I rebooted the device and this time it showed me the starting screen with all the greetings in different languages as if it was a new phone. I got through the setup (without logging into iCloud because it failed), but now my phone is really slow and very hot. I still haven't been able to see my messages on Kakao and Line (but I managed to get on Facebook through my computer at work). I also tried to back up my phone to my computer but iTunes says it can't connect to my phone (unknown error 0xe8000015) so I copied all the photos manually and reset all settings. Now it is still very slow and very hot, and I still can't log in to iCloud although through the web it is totally fine. Can someone tell me what to do? I feel like my phone was hacked.

Thank you.