deleted songs from iTunes

Hi

I upgraded to iTunes 11.0 yesterday. Today I noticed a couple of duplicate songs in my Classical genre. I went to delete these - a few duplicates deleted OK but then suddenly ALL my songs and albums in my Classical genre (a good number of them) just disappearred. I have shut down iTunes and re-opened it but no luck - all my classical music has gone!

How do I get all my songs back?
Thanks.