This is why you read so many posts about backup and restore of iTunes library and contents. You backup for the day something like this happens.
Make this as quick as possible. I've had a laptop for about a year and set it up without issue with itunes, running WIN7. Suddenly, Itunes on my "new" laptop is showing only 8 albums and their songs, about 10% of what used to be in the library. All of my music is in my desktop and iphone. I have syncd my iphone so many times it is absurd with nothing changing. I have turned on "home sharing", nothing. I have removed and reinstalled itunes on my laptop multiple times, nothing. I have authorized my laptop beyond multiple times, nothing. Itunes store purchases show up on desktop but only the 8 albums show up on laptop. I have performed so many recommended fixes I cannot even believe. I've never experienced anything like this. Any help out there?