This applies only to LCD displays, not older CRT monitors, and most people who have the problem which this trick can help fix are using laptops.
The wires which go from the computer part of your laptop to the monitor part of your laptop typically go through the hinged part of your laptop case. Constant opening and closing of those hinges, plus the inevitable dropping and other abuse of your laptop, can damage the wires and connected parts of your laptop, creating a black screen.
You can also get a computer black screen on your laptop if you spill something on it. The water or other fluid may conduct electricity, shorting out a vital component.
Whether it's wires or damaged components, you can often quickly figure out whether you need a new monitor, a new laptop, or just a small, cheap part using the flashlight trick.
The Flashlight Trick For Blank Monitors
Your LCD monitor has two essential parts: a Liquid Crystal Display (LCD) which turns white light into light of a particular color on each pixel of your display and a backlight which provides that white light. Older backlights were light bulbs, newer and high-end backlights are LED lights.
LCDs are crystals and they usually can't be repaired if they get damaged, but backlights are just LED lights or light bulbs-they can be replaced quite easily in many monitors (even laptop monitors). But before you try replacing the backlight, you need to discover whether or not it's the problem.
To test your situation, you need to replicate the backlight. That's quite easy: all you need is a bright flashlight with a white light (LED light blue color is fine too, but red doesn't work very well). You don't need to put this light behind your monitor, you just need to aim it at your monitor on a steep angle so the glare doesn't reflect back at you.
Now that you have a flashlight, turn on your computer and let it run through its normal boot cycle. When it gets to the point at which you would usually start using it, turn on your flashlight and aim it at your monitor. Look very closely at your monitor-if you can see the dim image of your normal desktop or login screen, then your LCD display is working correctly. It's just your backlight which isn't working.
Replacing The Backlight
If have a desktop monitor which hasn't seen any recent trauma (such as being dropped), you can usually just replace the backlight. Laptops are a bit trickier because the problem might not be the backlight-it might be another part, such as a shorted-out component from spilled water or a damaged wire from opening and closing your laptop too many times. If you're handy with a computer, you can replace your desktop backlight, but I recommend that you take your laptop to a repair shop so that they can determine exactly which part is causing your backlight to not work, causing your computer with a black screen.