Monitors malfunction for numerous reasons. When a monitor turns grey, it might indicate an incorrectly connected display cable or a faulty graphics card. Before assuming assume your graphics card might be the problem, however, check the monitor. Several interactions from the computer to the monitor take place to display a single image—and any one of these interactions could be faulty.
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Look at the back of your monitor just below the socket for a voltage selector. Set the voltage appropriate to your country, if it isn't in the correct position already. For a monitor in USA, the voltage is 115 volts.
Check the display cable to make sure it is properly connected with the metal connector engulfing the display socket on the computer.
Disconnect the monitor's display cable and look at its pins. If any of them appear bent, straighten them out by disconnecting the power cable from your monitor and sticking a thin metal object next to the pin and levering the pin until it appears straight again.
Adjust the brightness and contrast knobs on the monitor. The settings may have been at minimum or maximum; adjusting the knobs will help return the settings back to an optimal position.