I was an early adopter on IOS 4.2. The day it came out, I clicked "Check for update" in iTunes on the iPhone pane every five minutes.
After quickly snagging it, I quickly realized that it had caused a problem: When I opened the iPhone app, my music didn't show up! I was worried it had been deleted, but in fact it had not. I knew this because when I went to settings, general, and finally about, I showed 3302 songs.
Apple recommends a complete restore, but the problem with this is that it acquires you to go back and replace it a lot of settings on the phone.
A Simple Solution to the Problem
What I did to solve my problem as simply to go into my iTunes library and drag another song to my iPhone. This fixed the issue with little hassle and little time investment.
It's so simple that I'm surprised Apple doesn't recommend doing this first. If you try the solution, please let me know either way whether it works!