You will need
Check the monitor. Take the monitor and connect to another computer or laptop. If the signal is there — then it is OK. Check the cable from the system unit to the monitor. Connect the monitor to your computer, but this time use a different cable. If the picture does not change — so it's not in the cable.
Check to see if the computer starts. This can be done by ear — working computer makes a noise all of their fans are quite noticeable. But for further action auditory information is not enough, so remove the cover of the system unit, which opens access to the motherboard. If you press the power button inside the unit the computer is not alive any fan, the first thing you need to check the power supply.
Check the power supply. Take another power supply with similar capacity or exceeding it, and connect the connectors on the motherboard, as was done with the old power supply. The other components to touch yet not necessary. If the computer starts and the screen went to a picture, the weak link is found, and it's the power supply. If not, move on.
Check the graphics card. Pull the video card from the personal computer and insert another in advance making sure that it has a similar connector for video cards (PCI-E or AGP). If the graphics card has earned it to her disappear. You can put in your system unit with a known good graphics card. If video is still there, something is faulty, most likely the motherboard at least — the processor or RAM.
General diagnosis of the video card, if you doubt the quality of her work, you can check with the help of special programs: ATITool, 3Dbench, 3DMark05 and other similar. The Internet except for programs you will be able to find and detailed instructions for their use.