In my case, after upgrading to iOS 5 I had no music or video. But plugged into iTunes I could see 5gb of 'other'
When I clicked on Music in the device I could see all my music listed. I clicked on a track and pressed play and it worked. Then looked at the phone and it had all been miraculously restored (although I am having to reinstall most of it because the artwork in most cases won't show.
It was the same with the apps. Until I downloaded a new app, none of my existing ones would work. What a wonderful experience this iPhone upgrading is turning out to be!!!