Should your iPod decide it doesn’t want to show up in iTunes anymore, first reboot. Then try accessing the iPod through Windows Explorer. If you are prompted to format the drive, choose yes and format with all the default settings. When the format is complete, you should get prompted to restore your iPod (if not, then proceed to reinstall iTunes). You’ll end up having to add all your music again, but it’s much easier that trying to fix iTunes or any other worthless and time consuming process recommended by Apple support.