As excited as everyone was to get their hands on iOS 4.2 yesterday, it didn't come without a pretty ugly flaw. Several users were very unhappy to find that their music library had disappeared after installing iOS 4.2.
Fear not! There is a pretty easy fix that has been found by a crafty user on AppleForums by name of AppleZilla. Just connect your device to your computer via USB, pick a song from your library, play it via iTunes, click the Sync button, and then open the iPod application on your device. Your library will recover itself, and your tunes are now accessible again. Kind of a pain to have to deal with, but at least your library is not gone forever.