Check your iTunes Music Folder. Search & find it on your computer. See what the folder size is. You may need to re-add the files from that folder. You could open the music folder, select all the folders & drag & drop them to the iTunes Library icon at the top of the playlists whilst iTunes is open.

If the iTunes music folder is empty. Then you may need to get the iPod files copied back to your computer & add them to iTunes from there. See the above link in my first post & look for reverse iPod near the lower posts of that thread.

(I have been getting the impression that with apple adding different extras to their newer version iTunes that some of them are causing problems / side issues. Your iTunes music list might have been corrupted.)