Q: My Iphone 4s screen won't  turn on but phone continues to make sound?

Hi

I brought my Iphone 4S off of a friend and it worked fine there was a slight crack in top left corner of the pone but nothing major anyway i had it for about 3 days and the screen suddenly went blank. I then i plugged it in and it flashed on then went back off now whenever it's plugged into charge every 5 seconds it makes the charger connected noise and vibrates i've also noticed that the phone heats up an aweful lot faster while it is plugged in.Iim not sure what to do but any help/advice would be useful.