Hi,
Press and hold the Sleep/Wake button and the Home button (It's the round button located on the device right below the screen) at the same time for at least ten seconds, until the Apple logo appears.
Sometimes just pressing the home button will bring you back to the home screen regardless of where you are on your iPod.
If that doesn't work, try:
http://support.apple.com/kb/HT1394?viewlocale=en_US
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