You know this happened to me last night. I started out with a fully charged battery in the morning. Throughout the day, I had to be in a couple of areas where the signal was varying between very weak and none. Around 6-7 PM, the phone went blank. Before that, the last time I looked at it the battery was at 50%. So, I figured the low signal drained the battery to zero. When I cot in the car, I connected to the car charger and it did NOT come back to life. When I got home, I plugged it in to the wall charger and I still couldn't wake it up. I let it continue charging all night. This morning, Still no luck. So I pulled the battery and it booted up and has 100% charge.
Also, at some point while it was in that weird state, there was a period when the animated "Android" logo showed in a reduced state of brightness on the screen and eventually it went away.
Ahhh, the price we pay for technology. The good news was that I didn't miss any important calls.
Rob