Had same problem and solved it without having to reinstore any music or movies:
1. Delete old Library file
2. Go to the 'Previous iTunes Libraries' folder and copy one of the earlier files - note, they have a date extension, e.g. 'iTunes Library 2011-11-01'.
3. Paste this file in your iTuners folder and rename file by removing the date extension - 'iTunes Library'
You should have all your music and paylists in tact, unless it was a really old previous file.
Appears to be lots of similar solutions to this problem.
Search for iTunes Library on your computer.
From the list, select the most up-to date version of Library and save to folder.
Then delete iTunes from your computer and download the latest version.
When I did this, everything returned to pre-problem status. I didn't even have to re-import the saved Library file.
Hope this works for you.