Fix iOS 4.2ís Disappearing Music Bug
It seems that Apple is having a tough time ironing out all of the bugs in the recent iOS 4.2 update. First, the iPadís Wi-Fi connection issues that caused a delay in the release of the update and now another bug has popped up. Fortunately, this bug is more of an inconvenience than a real hindrance. Some people have reported music mysteriously disappearing on the iPhone and iPod Touch, right after updating to iOS 4.2. The music still appears on the iOS device when connected to iTunes but does not show up on the device itself. This appears to be a database bug, as the music files are still actually on the iOS device, but you just canít access them.
Fortunately, the solution to this problem is quite easy. All that is required to fix this bug is to plug in your iOS device and launch iTunes. Then in the sidebar, click the arrow next to your device and select music. Play any song and sync your device again. When you go to play a song on your iPhone using the iPod app, all of your music should be available for listening.
This is an easy solution to a problem that should never have existed in the first place. Hopefully Apple will release another update soon to take care of this problem. A few more beta testers would probably be a good idea as well.
Source: 9to5 Mac