IPod Doesn't Appear in iTunes
Sometimes you connect your iPod to your computer but you can't see it in iTunes. (This happens to me once a week or so at the office!) From iPod appears in Windows but not in iTunes (abridged):
Install the latest version of iTunes
Restart the iPod Service
Remove and reinstall iTunes
Empty your Temp directory and restart
Disable conflicting System Services and Startup Items
Update, Reconfigure, Disable, or Remove Security Software
Sometimes you worry that you'll DELETE your music AGAIN, or you run out of space on your hard drive. From How to re-create your iTunes library and playlists:
...Actually, just click that link. Apple provided step by step directions that I would otherwise type out or copy and paste.
Make sure you back up your library before you need to re-create your library. I use an external hard drive to manage my music. This doesn't guarantee safety, but it has made things easier.
How to back up your media in iTunes (with a CD or DVD)
It's always good to be proactive. From Apple's How to back up your media in iTunes:
...And Apple's done all of the work for me here, too. Click the link for more. Apple also explains how to restore from the disc you just made!
iPod appears in Windows but not in iTunes
How to re-create your iTunes library and playlists
How to back up your media in iTunes