How do I fix my iPod if it won't show up on my computer?

Apple provides several support documents related to this issue, depending on the symptoms and the platform used.

Mac users may find these iPod documents useful:

Not on Desktop or iTunes
Not on Desktop or iTunes & "!" Icon Onscreen
On Desktop, Not in iTunes
On Desktop, Not in iTunes, or iPod Updater

Windows users may find these helpful:

Isn't Recognized
Isn't Recognized on Laptop Over USB
Software Not Installed Message When iPod Connected
Computer Won't Start When iPod Connected

For solutions to other specific problems, please refer to the iPod Troubleshooting & Repair Q&A and try the iPod Help links provided for each iPod model on EveryiPod.com.

If after going through all of the above, the issue appears to be a hardware problem, site sponsor Mission Repair offers free hardware diagnosis and an affordable 24 hour repair service by mail for all iPod models.