Why doesn't my iTunes work anymore after trying to update it?

Yesterday I tried to update my iTunes to what I believe is 8.1.1, and at the time it said that either the .itl file is not supported or it was damaged. It won't let me open any of my iTunes, and the shortcut on my desktop was deleted. I tried to re-download the iTunes, and now it's telling me that I'm not authorized to download it. Does someone know what's going on?