not sure how many people have this problem after they upgraded their iphone OS to 4.2 but it seems there are many people have this problem after their iphone's OS got upgraded. I found out there maybe a solution to fix it, so i post it to here. for details please read this link from Simon Cohen:
http://www.sync-blog.com/sync/20 ... s-ipod-content.html
please post more solutions if there are other ways to fix any problems after the OS has been upgraded. Or if there are other problems after the OS has been upgraded.
a solution from the above link:
"Thanks to reader jonoo who suggested re-connecting the iPhone to the PC and playing a few tracks in iTunes FROM the iPhone’s library, then disconnecting the iPhone. It works! All tracks are now back.
Let us know if this fix does not work for you."