Java Programmer

Chances are that if you own an iPod touch, iPhone or iPad, you jumped up to get the latest iOS update. iOS 4.2, in addition to introducing multitasking to the iPad, brought along a little bug that's making music collections vanish into thin air. A number of iPhone 4, 3GS and iPod touch owners have complained that, after iOS 4.2 brings multitasking from iPhone and iPod touch to iPad. So you can be more productive as you work, have more fun as you play, or do a little of both. Everything happens smoothly and efficiently — without slowing down the performance of the foreground app or draining the battery unnecessarily.²
updating to 4.2, the iPod app displays a message reading "No Content" (even if the files are still there, floating about in iOS purgatory.) Thankfully, the Switched team and their precious playlists have escaped the issue, but TechCrunch's MG Siegler couldn't locate any of the 7.8 gigs of music on his phone. After some forum digging, however, Siegler found that there's a relatively simple answer to problem.

Just plug your device into your computer, play a song that's stored on your iPhone or iPod through iTunes on your PC, then resync. Disconnect, and voilà -- your music will have magically reappeared when you open the iPod app. No need to fire off obscenity-laced e-mails to Steve Jobs.