Without doubt the iPod by Apple has become one of the most iconic symbols of 21st century consumer electronics.
The latest iPod Nano (4th Generation Chromatic) is a funky little device, but after uploading an mp4 movie onto my son's green iPod it froze during the middle of playback. (about an hour in)
After consulting many forums on the web, I managed to reboot the iPod and it worked normally afterwards
The method I used involved holding the top of the click wheel (where it says "MENU"), and the centre of the click wheel (or SELECT button) together for around 10 seconds. After this the Apple logo displays and all is well from then on.
If this doesn't work, then there appears to be a simple last resort solution. Just llet the battery run out! After it has run dry, re-plug it in to the USB lead from your computer to recharge and reboot!