The same problem as you had. Details of the experience I had may help. After installing the latest iTunes, two months ago, I found that all the songs showed, as did the albums. On approaching a song in any playlist, the screen showed an exlamation point(mark in UK) to the Left of the song name, and asked me if I would like to look for it. I'm sure this can be done, but with 11,000+ songs I will have no time to eat. I am going to approach this very carefully. A year or two ago, after a similar road-block, I re-assembled 500+ songs into their Playlists over many hours, but on this occasion would very much prefer a quicker, ultra-secure solution...glad, though, that you have fought your way through successsfully. The Repair Disk suggestion from mallory623 is tempting.
BPOFan