I had the same problem. Screen was black until i plugged it into a charger from an outlet, then logo kept going on and off. Tried all teh fixes to reset and restore through itunes, but nothing worked. I found a little info on what could cause problems with an iphone rebooting and took a guess that it was the battery. Had the battery replaced and it's fixed. It cost me $25 to get the battery replaced on an iphone 6S, but prices can vary greatly depending on the repair shop, so call around.
Topic:
App & System Services
SubTopic:
Hardware
Tags: