Why won't my phone stay on when it is not charging?

I left my phone on the charger all night. When I woke up I unplugged it and it was Hot!!. When I unplugged it, It shut off and would not ome all the way back on unless it was charging. I tried taking out the battery and letting cool down but still no luck. I already checked for water damage and it fine. What should I do?? Please help..