You may need to shut down your computer and reboot. During reboot hit F8 or the Del button to go to setup(It is different on each machine but look for the prompt on the screen that says setup). When in setup look for safe mode with video as an option and hit ok. Your computer should now boot up in safe mode with basic video capabilities. My suggestion from here would be to go to Control panel/Display and reset your monitor the way you had it. Restart from there and this may do it.
mike
April 2008