iTunes Software Update Failure

The new iTunes software update failed to install and now I can't access iTunes on my computer at all. I read that I should try to re-install iTunes but that doesn't work either. It tells me that the iTunes.msi isn't available on my computer. I am not sure what to do. I tried to roll back my computer to a week ago when iTunes worked and that hasn't solved the issue. When I try to install iTunes it asks for permissions to my Apple mobile device. I click "ignore". I am not sure what is going on with this. Please help!