What happened to my iTunes movies and music after I upgraded?

I am a long-term subscriber to iTunes and have purchased many music albums and movies. I would like to find out what happened to my iTunes movies and music after I upgraded. My ENTIRE INVENTORY of music and movies VANISHED! So what must I do to reclaim all my purchased music and movies?? This is nothing short of devastating! I use Itunes for Windows. Now I am adrift, and very confused!

[Re-Titled by Moderator]