Follow these rules might help you
Is the power cable for the monitor plugged in correctly? Is the monitor turned on? Is the cable from the video card to the monitor plugged in correctly? Is the video card seated properly? Take the cover off of the case, remove the video card and insert it again properly, then try again.
If everything is connected and powered up correctly, push the power button again and pay close attention: do any lights on the front of the PC blink or light up? Does the fan in the power supply start whirring? This could indicate a hardware problem. Try to think what happened right before the problem started. Did you add a new piece of hardware? If so, it might be faulty or not installed right. Remove it, then try again. If everything is fine without the hardware, try to reinstall and configure the hardware again. If that does not help, try exchanging it and see if that corrects the problem. If the error persists, narrow it down systematically. Remove everything but the CPU, memory, keyboard, and video card, then boot again. Does the error still occur? If not, then add one piece of hardware and try again. Repeat until one new component causes the error and you now know who the culprit is. If yes, then reseat your CPU, memory and video card and try again. If the error still happens, try replacing the CPU, video card and memory one by one to see if one of them is the troublemaker. If that does not help, you could have a bad motherboard. Check the motherboard carefully for any signs of damage. Also check for shorts, a metallic object could have found its way onto the motherboard, or the motherboard is not installed correctly and touches the case somewhere.
Another possible cause could be incorrect CPU settings. If you replaced the CPU with a different one, be sure to choose the correct settings for multiplier and bus speed, either via jumpers on the motherboard or a menu in the BIOS. Consult the manual for the correct setup. If you can’t even get into the BIOS to change the settings, use the jumper on the motherboard to clear the BIOS and load the default settings, it should allow you to