Before diving into the more technical stuff, first check the easy fixes.
1. Is the power switch on the back of the computer turned on.
2. Is the power cord plugged into the back of the computer.
3. Is the power cord plugged into the wall.
4. Does the wall outlet have power to it.
5. Check the voltage selection switch to make sure that it is set for 115v (for US)
Is it plugged into the mains.
Is there power to the machine.
Does the machine actually start when you switch it on.
Does it pas the POST (Power On Self Test) function.
Is the hard-drive spinning.
The first thing asked is always the most obvious of Questions.
1) Is the Computer plugged in?
2) Are all the Connections properly connected?
3) And the worst of all, but knowing some people: Have you turned the Computer on?
Some people think a computer should just turn on once it's plugged in.
Sometimes a computer won't boot because it got fried, so ask if the Computer is plugged into a surge Protector and if there were any electrical storms recently.
It could also be because of a Virus, there are many reasons why a PC won't boot...
Is the computer plugged in?
When did this first happen?
Were there any unusual behaviors before this happened? What has changed recently?
How old is the computer?
Will the computer not turn on AT ALL (no lights/fans/noise), or does it turn on with no video (black screen) and make no noise?
If it turns on and makes noise, what is the noise?
Is there anything onscreen, and if so, what?
Just a few examples. In general you can condense any troubleshooting situation down into the PAIR process:
Probe, Analyze, Inform, Repair
Probe: Ask questions to find out what, exactly, is happening.
Analyze: Analyze the information to form a hypothesis on what has gone wrong and how to fix it.
Inform: Inform the customer of your hypothesis and propose a solution.
Repair: Once agreed, attempt the solution and see if the problem is repaired. If not, repeat.
One good question to ask is if all the cables are plugged in. Another question to ask is if there are any lights that are on.
BIOS
A driver
May be the OS has been lost/erased/deleted and you might need to Reinstall a OS
A hard boot, or cold boot, involves turning on the power with the on/off switch, A soft boot, or warm boot, involves using the operating system to reboot.
A a soft boot takes the intials steps if a hard boot doesnt happen.. A hard boot initializes the processor and clears memory
BIOS
hard drive failure
over the network
Boot from BIOS n use VGA mode only
Recovery Console
A driver
BIOS
windows recovery console
The Windows Recovery console: http://support.microsoft.com/kb/314058
windows recovery console
Reboot the system, press and hold F8 until a menu is displayed, and then select Safe Mode
start the computer in safe mode and roll back the driver