To help protect the computer from improper shutdowns while it is running, the power button will not turn off the computer. If your computer is hibernating or experiencing other problems and needs to be turned off, press and hold the power button without letting go for 5-10 seconds. Holding the power button allows you to turn off the computer.
Tip: While a computer is running, pressing the power button may put the computer to sleep. When a computer is in sleep mode, power indicators usually flash and pressing a button on the mouse or keyboard wakes up the computer.
Finally, if you have a laptop computer and followed the steps above without turning off the computer, unplug the computer and remove the battery.