Try detaching the hard drive from the logic board, then reattach it. Link text
When the iPod is starting up, do you hear clicking? This is a sign the hard drive is failing, or has failed.
Try restoring the iPod by putting it into Disk Mode: http://support.apple.com/kb/HT1363
If none of those work, then try placing the hard drive in the same iPod model (if one is available) to see if that working iPod sees it... And vice versa.
If the hard drive is good, then you can bet it will be the logic board that needs to be replaced. Make sure you replace it with the same model number: 820-1763-A or 820-1975-A.