O.K. that was a try. Did the same thing, returned to login on my DVI port.
Then you are using the VGA port.
And there is no beeping after you login when the screen is black? Just checking.
So, how did you set up your installation?
Did you set this PC to be a core only or a hybrid?
Did you set it up to act primarily as a PC - Ubuntu starts by default, or, as a dedicated LinuxMCE box?