Some iPhone owners found that after upgrading to iOS 4.2 their iTunes music libraries were missing from their phone. Odds are your music isn’t really lost, and doesn’t even need to be reloaded.
iOS 4.2 can make music lists like this disappear
If the iPod app on your iPhone thinks you don’t have any music loaded after the iOS 4.2 update, just reconnect your phone to your computer so iTunes can perform a normal sync. Since your music is probably still on your iPhone and the iPod app is merely confused, the sync should go quickly because there aren’t any tracks that actually need to be added back.
Some users have reported that they need to play a track in iTunes while their iPhone is connected to their computer, but none of our tests required the extra play-a-song step.