There’s no easy way to determine exactly why a PC won’t boot (we’re assuming that you tried booting from an optical drive or a bootable USB disk, and it failed), but the following steps will take you through the most common hardware reasons for a PC that won’t load Windows.
Check all of the external cables, including the power cable. Confirm that your monitor is turned on.
Do you hear beeps while the PC tries to boot? Write down the number or the sequence of beeps (for instance, one long, three short) and search the Web to learn their meaning. If you know the manufacturer of your BIOS, that will simplify the search. For example, try a search for “Phoenix 1-2-2-3 beep code” (for a PhoenixBIOS PC with a pattern of one beep, two beeps, two beeps, three beeps); in this case, you’ll see that the PC has likely experienced a motherboard hardware failure. Even if you don’t know the BIOS maker, you should be able to determine the issue this way.
If your PC doesn’t produce beeps, open the case and verify that the internal cables–especially cables from the power supply and cables linking the hard drive to the motherboard–are properly connected.
Make sure the RAM is properly seated; remove and reinsert it, and try booting with one RAM stick at a time (if dual RAM sticks are not required, of course). If you have a spare RAM module or two, try subbing them in.
Your system’s power supply may have gone bad–many are cheap and prone to failure. Try connecting a spare power supply to your system (you needn’t install it inside the case), and see if the PC will boot. Don’t be tricked by whirring fans: A power supply may have enough juice to keep the fans going, but not enough to power everything else.
If your PC has a discrete graphics card, try removing it and attaching the monitor to the integrated graphics connector.
Finally, if all of the above steps fail, you’re probably dealing with a bad motherboard or (less likely) a fried CPU.