What to do when your computer will not POST

POST stands for Power On Self Test. It is the routine the BIOS runs immediately after the computer powers on and before the computer displays the screen with the company logo or BIOS information on it. If there is a serious problem with the computer's hardware, the POST can hang or produce error tones preventing your computer from booting. These are the steps you can use to diagnose your problem.

NOTE: If your computer's fans do not spin when you press the power button, your computer is not powering on. This guide does not address that issue, yet I hope to have another guide for it soon.

NOTE: To prevent damage to your computer, do not touch, remove or install hardware while the computer is plugged in and do not work in the inside of your computer while standing on carpet flooring. This can cause static electricity buildup to damage boards.

Sometimes you may get beep codes when you try to start the computer. This website has a few of the common ones depending on your BIOS.
BIOS beep codes

One common and easily fixed cause of no POST is loose RAM. With your computer unplugged, take your RAM out and put it back in. This will ensure that the RAM is secure in its brackets. See if your computer will boot now.

If your computer will not boot, it is time to start the process of elimination. The point is to eliminate each piece of hardware as the problem. The easiest thing to test is removing optical and hard drives. If this does not fix the problem, try removing pci/agp cards. If removing any of these devices causes your computer to post, you should narrow down the specific device by re-connecting devices one after another. When you find the offending device, make sure it is not data or power cables causing the problem.

At this point, if your computer does not boot, you should be down to the power supply, motherboard, processor and RAM as possible causes. You should replace the power supply and RAM with known good devices to test them. If replacing RAM helps, make sure that it was bad RAM that was the problem and not a bad RAM slot. If at this point, your computer still will not boot, it must be your motherboard and/or processor (probably processor). At this point, I would replace both, because even if you find out one is causing the problem, it might be the other that damaged it.