I downloaded the latest Itunes Update, then I got an error message. It deleted my entire Itunes library. Any suggestions or comments?

Hello,

I downloaded the latest Itunes Update and got an error message. When I clicked ok on the error block, my entire Itunes Library was deleted. Fortunately, I have most of my music backed up on a spare hard drive. Anyone know the reason for this, and if I discover I am missing some music files will Apple replace them?