iPod Classic – 1. Toggle the Hold Switch on and off (set it to Hold, and then turn it off again). 2. Press and hold the Center and Menu button for 6 seconds, until the Apple logo appears in the center of the screen, the iPod will now restart.
iPod Touch – Press and hold the Home button and the Sleep/Wake button at the same time for 15 seconds. Once the apple icon appears, release the buttons and the iPod will reboot.