Iphone overheated and not turning on after cooling down

My iphone overheated and froze. It turned off and i let it cool down. After an hour i tried to turn it on but it wouldn't. The phone was cool and i even tried to force restart it but it doesn't respond. What could be the problem and how to fix it?? Why wouldn't the iphone turn on after cooling?? I charged it later and the charger doesn't seem to get warm while charging but the devices gets warm. Forced restart doesn't seem to work either. Can someone help??