Why won't my phone turn on and why is it getting so hot?

I was on my phone just a few minutes ago so I put it down and went to go get a snack because I was bored, but when I got back my phone wouldn't turn on. I took it off charge and held my phone and I realized it was starting to get really hot. I wasn't even on my phone for that long and it seemed fine before I got up. I'm trying to let it cool off but it doesn't seem like anything is happening.