After you update to iOS 4.2.1 on your iPhone (or iPod touch), check to see whether your synced audio and video content is still there. Many users (including myself) have been dismayed to find that all the content was missing. If this happens to you, stay calm. Your content is still there.

To get everything working again, reconnect your iPhone to iTunes. At this point, you should happily see that your media content remains listed on your iPhone. You won’t need to re-transfer any data. You just need to help your iPhone find the data that’s already there. To do so, click to sync the iPhone. When done, the missing content should be “restored.”

Some reports claim that, before this resyncing works, you’ll need to select to play a song from the iPhone’s media list in iTunes. I did not find this to be necessary.

(Thanks to , where I first spotted this tip.)