If an Apple iPod is locked up or continues to display the “Do Not Disconnect” message after being disconnected from the computer, press and hold the following combination of buttons for approximately 10 seconds to reset the iPod.
Upon reset, your will see the iPod/iPhone name and the Apple logo. When this happens, you can let go - the iPhone/iPod is rebooting. If your battery is very low, you may have to attach your iPod/iPhone to a power source.
NOTE: If you still can't get your iPod to restart, try placing it on a table or flat surface. Make sure the finger pressing the Select button is not touching any part of the click wheel. Also make sure that you are pressing the Menu button toward the outside of the click wheel, and not near the center.