ok lets understand what you have done:
you said "restart". Did you FACTORY RESET the phone or just tap the "restart" icon on the shutdown screen?
its going to be next to impossible to understand what happened, especially over the internet.
once certian ingredients mix under the hood, one quick tap on a screen or one more app to try and run could have pushed it over the edge,
so lets just cal it a "glitch" ok?
your main focus is getting the phone on.
try some button pushes:
press and hold the power button for about 30 seconds (no you dont have to stand on one leg or put tin foil on the cat).
just unplug it and remove external power from it.
try power and UP button at the same time also. again holding for a number of seconds...