Dumb question but have you tried charging it? Not trying to offend you with that question, it could've died. Also try taking the battery out and putting it back in it again and attempting another restart.
I had an issue the other day where the battery indicator somehow froze so I didn't know how much was left until right before it was dying. I had to shut it off, remove/replace the battery, and restart it again plugged in for the battery indicator to become accurate again.