I had a similar sounding issue with my computer, but it was prior to running
Windows update. Essentially, the driver
for my Radeon 9800 (supplied as part
of Vista) was broken, and every logon, unlock, and UAC screen I received
would blank the screen and turn the monitor off (like in your description).
To resolve this (as I set a password on my account), I would just have to
enter the password and press "enter", or in the case of UAC press "C" for
continue completely blind. Once the desktop session loads, the display comes
If your computer is on, but the screen if off, I would suggest trying to
navigate blind and performing a video card driver update as soon as you can.