Unlike with a desktop computer, it’s not that simple to turn off your laptop when it freezes up or misbehaves. You just can't yank that power cord from the wall on a laptop. The reason that it doesn't work is that with the AC power gone, the laptop immediately starts using its battery. This can be very disconcerting when the system is locked up and you really, badly, want to turn the sucker off.
If the computer just utterly seems to be ignoring you, press and hold the power button. Keep holding it down, usually for 5 to 10 seconds. Eventually, the laptop turns itself off.