|If this happens while booting the computer BEFORE the operating system loads from the hard drive, that can't be caused by any software on the hard drive - that's a hardware problem.|
If it's an external monitor, try it with a different computer, and/or try a different monitor with the problem computer.
If it's the built in display on a laptop computer, try it with an external monitor.
Laptop no video (or video problems), and battery, AC adapter, power jack T shooting.
See response 1:
DO NOT plug in or unplug a monitor's video cable into / from a video port while the computer is running. Doing so can damage the monitor's circuits and/or the video adapter's circuits.
Some laptops will display on an external monitor without you having to do anything.
If you get no video on an external monitor, some laptops have a key combo that you must press to toggle the display to external video only / built in display video only / sometimes both - see the Owner's or User's manual for the model.