Here are a number of possibilities:
1) Under 'Extension Manager' in your Control Panels make sure the following are turned on:
FireWire Enabler
FireWire Support
iPod FireWire Enabler
if they aren't turn them on and restart your computer.
2) If you have any other FireWire devices plugged in unplug them so only the iPod is connected.
3) Make sure the iPod is not 'locked up' (the hold button is on).
4) If none of those work try using a different firewire cable.
5) If none of those options work, do you have a friend whom has a mac? If so try plugging the ipod into a different mac (you could probably go to a local Apple retail store to check it out)
Those are the only things I can think of. If you find that it works on someone else's mac but not yours, you probably have a bad firewire bus or logic board. If it doesn't work on any of the macs you try it's probably a bad HD or FireWire port on the iPod. Good Luck!