Killed my phone by USB tethering
USB tethering by all means should not cause this and I do not believe that your battery is dead. Instead, it's more probable that you stumbled upon a glitch within the OS.

Back up all of the data in your phone including photos and other things in your external storage, then do a factory reset on the phone. If this does not resolve the issue, there may be more serious underlying hardware or software issues. Contact the manufacturer if a factory reset does not fix the issue.