After updating my iPhone 4 to iOS 4.2 yesterday, I noticed a strange thing: none of my music was in the iPod app where I left it. At first I figured that it must have gotten wiped in the mysterious update process, but upon reconnecting my iPhone to iTunes, it was all still there.
It seems I wasn’t alone, and I stumbled upon this 9to5Mac post outlining and offering a fix for the issue. That simple fix, from Apple’s support discussion boards worked perfectly for me, but might even be overkill: simply re-syncing your iPhone (connecting it to iTunes and pressing sync) should fix the problem.
If that doesn’t work, there’s one extra step (from the support discussion) to try:
Connect your iPhone to iTunes and click on Music under the device name in the right sidebar. Your music should be there; if not, you may have inadvertently removed it in the update process (possibly by restoring your phone to factory settings). Assuming it’s all there, select a track from the iPhone’s music list and play it through iTunes (a second or two should do). Then re-sync the phone, disconnect it, and open the iPod app.
Like magic, your music is (hopefully) back!
Let us know which method works for you in the comments.