1. Check that your hold switch is off by sliding the switch away from the headphone jack.
2. Press and hold the following combination of buttons buttons simultaneously for approximately 10 seconds to reset the iPod.
iPod 1G to 3G: "Menu" and "Play/Pause"
iPod 4G+ (includes Photo, Nano, Video, and Mini): "Menu" and "Select"
Shuffle: Turn it off, turn it on. Done.
If you have an iPod Mini or 4th gen iPod or higher, then you need to press the Menu and Select buttons, not the Menu and Play button, to restart. The Select button is the center button in the middle of the wheel. Upon reset, your will see the iPod name and the Apple logo. You may have to attach your power plug in your iPod to make this work. Your iPod should mount on your computer now. If not, keep reading.
3. The iPod name and the Apple logo will briefly appear upon a successful restart before advancing to the Main Menu.