Missing music files in iPhone 4 after iOS 5 upgrade

I have upgraded my iPhone 4 into iOS 5 just the other day. However, after upgrading, I noticed that most of the album artworks in my phone were missing. It bothered me a lot and I wonder what I did wrong in iTunes. So I tried deleting an artwork from a song, then put it back but still it didn't work. Worse, all of my songs mounted in my iPhone before they disappeared, even in iTunes. I can only see them in the Library. Apparently all of my songs have an icon with "!" beside them and I cannot locate them anymore. One more thing, the capacity bar shows that 5 GB size of audio is still used. What could possibly have happened? How will I be able to retrieve my songs? Help me please.
