A little more detail on how to power off since it is not as simple as pressing the power button which actually just puts it to "sleep" or in "standby" by turning off the screen and some other stuff.
Powering off means just that, turning the device all the way off. To do this you press and hold the power button until a menu is presented. The contents of this menu vary by device, but they all include a "Power off" option (or the equivalent). Some also have a restart option to power off and power back on.
Powering off literally shuts the device down and turns all the electronics off. Nothing will wake it up until you do. Rebooting from this state takes quite a while. In desktop computer "lingo" this is called a "cold boot."
Cold booting can solve problems with something that's "gone wrong" in RAM and can solve many problems so consider doing this when you're getting inexplicable Force Close type errors.