For iPod Photo, iPod mini and the iPod with the click-wheel, toggle the hold switch on and then off. Very quickly, then press the menu button and the selection button at the same time. Hold them until the screen resets (usually about 6 seconds).
For all other iPods, toggle the hold switch on and then off. Very quickly, then press the Play/Pause button and the Menu button at the same time. Hold them until the screen resets (usually about 6 seconds).

If this does not work right away, try plugging it in and then repeating the above steps while the iPod has a power supply.