ITunes library GONE!?

I opened up my ITunes to listen to music and all of my music and playlists were completely gone. I plugged in my IPod and a message popped up that said it was synchronized to a different computer, which is strange because this is the only computer I've ever used for my IPod. After searching the web, I found that most sites said to look in my files for the "ITunes Music Library.xml" and open it in ITunes. There are many of these files on my computer, none of which contain any of the music I've uploaded. I don't know what to do and I'm freaking out. Thousands of songs gone! Please help me! What do I do?