My phone was at 100% and won't turn on, any help?

I charged my phone early in the afternoon and fell asleep to my phone being 100% on battery, I unplugged it and just sent 1 text and after about 2-3 hours later, my phone was warm and it wouldn't turn on. I tried the home + sleep button thing and it turned back on. why did my phone just decide to die even though it was at 100%?