Explain what happened during the attempt at jailbreaking...I don't see how this could happen without you ejecting the iPod mid-jailbreak. If this is the case, try re-jailbreaking. If you cannot do this, try this:
Plug into iTunes.
Turn iPod off.
Hold down power and home buttons at the same time until it restarts.
After it restarts and the apple logo shows up, release the home button but keep holding the power button.
This should make iTunes recognize your iPod so it can be restored.
If neither of these work, Im afraid your iPod is toast.