If any malfunction turn off the computer, remove the lid to see its contents. Visually inspect the motherboard. Verify the absence of gross mechanical damage. Check for swollen capacitors, charred parts. With a magnifying glass inspect for scratches and gouges on her tracks.
Your further actions depend on external signs of malfunction:- when you turn on the computer does not submit signs of life, go on to step 3;
- if you turn the computer on the fans run, LEDs lit on the motherboard, but the monitor stays dark and the PC speaker makes no noise, proceed to step 5;
- if the system speaker emits one short beep, then most likely the motherboard is OK, and the reason is the video card or the monitor;
- if you turn the computer normally boots, but then becomes unstable, i.e. sometimes spontaneously freeze or restart – go to step 9.
Eliminate the fault in the power supply, for example, replacing it with a known good.
Pull back and reinsert the cable connection between the power supply and the motherboard. Turn on the computer. Maybe it will work, and the reason was a bad contact in these connectors.
From slots unplug all devices except video card. To distinguish between a video card is easy – the cable from it goes to the monitor. Disconnect from the motherboard and power supply, hard drives, CD and DVD drives, drives, floppy drives. Disconnect all external devices except monitor and keyboard.
Turn on the computer. If the monitor displays an image, go to step 7, otherwise – to the point 8.
Turn off the computer. Connect the hard drive. Turn on the computer. If the monitor is dark again, the device is faulty and requires replacement or repair. Replace it with a serviceable. Complete this item for all previously disabled devices. It may happen that after the return of all devices in place the computer will function normally. This means that the reason was a bad contact of one device.
If after removing all the devices except the video card, the monitor remains dark, check the CPU, memory, and video card. You can check them out, setting into a working motherboard. Risky to do the check Vice versa, i.e. to insert into your suspicious motherboard serviceable device, this may bring them down.
Also, instability can be caused by overheating of the elements of the motherboard or power supply. Check that all fans are spinning normally, and the cooling radiators in the process, do not heat up much. For this you can turn off the computer and just touch them with your finger. They can be warm, but no more.