Please ! Help!

Hi everyone and anyone. I have a major geek problem. I was playing a game and also downloading another game from wild tangent. I turned to speak to someone in my room to talk to them and when I turned back to the PC it was a blank black screen. I tried turning it back on and I do hear it come on and the on light is blinking as usual but the screen is still black. It's a Dell PC and is less than 3 years old and I've never had a hint of a problem. Any ideas to getting it to come on again? Many thanks!