Pull the Power Cord.
Press the Power button.
Press Start on a PC, Special on a Mac.
Press the power button on the Power Switch.
Run a couple of high powered magnets along the frame of the tower.
The help file
Who was the last person using this computer?
Forget this, I'm calling Technical Support.
Make sure everything is plugged in correctly and turned on.
Touch Screen Monitor
Purchase and install another hard drive.
Add more memory.
Compress the hard drive.
Format the hard drive.
Hook up a second monitor.
The monitor cable may have come loose.
The government may be sweeping your computer looking for terrorist activity.
Your neighbors may be playing "Rock Band" too loudly again.
The dead may be trying to communicate with you.
Your monitor may have run out of color pixels. It is time to bring the monitor in to your service tech to have the color refilled.