Cause: The display controls could have ‘pushed’ the image off-screen. It may also be a problem with the refresh rate or the resolution being set too high.
Solution: Adjust the display controls and see if you are able to get back a stable picture. Reduce the refresh rate and/or the resolution. If you are not able to see the screen, reboot and press [F8]. Select Safe Mode from the list of options; Windows will start with a simple driver at a low resolution and low refresh rate. Disable any video drivers (through Device Manager) so that Windows is forced to use its own driver to handle the display when you reboot normally. After rebooting in normal mode, enable the driver and run the Add New Hardware Wizard to redetect your video driver at the basic resolution. Readjust your refresh rates and resolution to below the values that caused the scrambling.