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A number of users are reporting that after upgrading their mobile devices to iOS 4.2.1 the Music app no longer shows any songs. It turns out that the update doesn’t actually remove those tracks — it just hides them in some cases, for some reason.
I didn’t have any problems with iOS 4.2.1 hiding the songs on my iPod touch. But if you do have a problem, it looks like there’s a five step plan that can help:
1. Connect your iOS device to a computer with a USB cable.
2. Click the arrow next to your iPhone or iPod touch in the “Devices” section to expand the menu.
3. Select Music.
4. Play a song from the list on your computer.
5. Sync your device again.
That’s it. Once you’ve gone through those steps, all of your missing music should show up again in the Music app on your iOS device.