PC working fine all of a sudden it was off

Hello,
I have a personal computer (Desktop) at home. It was working fine and all of a sudden the computer was off and when i try to restart the computer the monitor is swithching on and off and displays a message as "No Signal". and the DVD light is glowing on and off. The screen is blank. and if I press some other key it says as press F4. when I press F4 nothing is coming. Could some one help me out to fix this issue please.