However, users who have upgraded to iOS 4.2 are reporting that the update is wiping out all their music/songs and launching the Music app displays a message saying "No Content" available. This message can come as a shocker to many, but fortunately your music has not been wiped out and there is a quick fix for this issue.
TechCrunch has pointed out to a simple fix for this issue. If you see "No Content" when you launch the music app on the iPhone, iPod Touch or the iPad, just plug the device into the computer and play a song from the device. Once you have done that sync your device and your songs will be recognized again.
So don’t panic, your music is still there alive and waiting to be heard.