The computer start-up process can be divided into three basic steps:

Power On Self Test

BIOS

Boot (or Bootstrap)

Troubleshooting system start-up problems can involve looking at one or more of these steps.

Although many things can go wrong with a computer, perhaps the most frustrating is when your computer won't start at all. This is true, in part, because when your computer fails to start you cannot use diagnostic tools to help you determine the problem.

If your computer won't start at all, the power light does not come on and the cooling fan is not running:

Check if the wall outlet you are using will run another appliance such as a small lamp.

Change the wall outlet you are using -- perhaps there is inadequate voltage.

Check to make sure that the computer's electrical cord is securely plugged in at both ends. Switch it with another cord if you have one available to you.

If none of these steps help, it is likely your power supply has gone bad. Power supplies are potentially dangerous units and a professional should replace your power supply if you need a new one. Never open up a power supply box.

Having a bad power supply can be the cause of many problems that may not initially point to that component. For example, spontaneous reboots or freezes and even memory parity errors can be traced to faulty or inadequate power supplies.

It is almost always easier, and often cheaper, to replace a defective power supply rather than repair it. If you do have to replace your power supply, be mindful of the physical requirements (shape, location, screw-hole positions and so on).

If there is no power light, but your cooling fan is running:

Perform the three steps listed above.

Open up your computer and make sure that your 6-pin power supply cables (there are two) are securely attached to the motherboard (note that such 6-pin cables are unique to your system's power supply harness).

If none of these steps help, you likely need a new power supply.

If your power light is on and the cooling fan is running, but there is no computer activity (you hear no beeps at start-up):

Open your computer and make sure your power supply cables are securely attached to the motherboard.

Examine your motherboard for metal pieces that might be touching other metal pieces (such as screws or the motherboard's seating). These could be shorting out your motherboard.

Check the seating on all the other expansion boards in your computer. An expansion board that is not seated securely can cause a short.

Make sure all other cables inside your computer are securely seated and in good condition. Look for frayed or crimped edges.

Check that your CPU is inserted properly in its socket, that it is cool, that the CPU heat-sink/fan works and that it is fitted correctly.

If your CPU has a tension lever, make sure it is closed and locked.

If you hear two or more beeps at start-up, but there is no video: