Some computer devices can mysteriously stop working, or you may have difficulty installing them in the first place. If you are having trouble getting an image to show up on your monitor's screen, there are several things you can check before taking the problem to a technician.
The most common cause of a lack of signal is merely a loose or missing cable. Your monitor needs a power cable plugged into the wall and a video signal cable connecting the monitor to the back of the computer where it attaches to the video device.
Just like a TV, a computer screen has a power button you need to hit to activate it. You or someone else may have bumped into this button and switched the monitor off. This button will usually be lit with a green light when the monitor is active.
Sometimes a power or signal cable is securely attached, but you're not getting an image. On occasion a signal cable's wires get loose or frayed if the cable has been regularly twisted to fit around corners and through small apertures in a desk; in this case, it needs to be replaced.
Faulty Video Device
It's possible that the video display device in your computer is faulty and needs to be replaced; or one type of output no longer works. If you're using VGA cable (usually with blue ends), try switching to a DVI cable (usually with white ends) if your monitor has a DVI port.
If your monitor supports VGA, DVI and other types of visual input, you might just need to switch to the active input to get the signal, like changing channels on a TV. Refer to your monitor's documentation for input selection.