Check to see if the cable is correctly inserted properly. Make sure the power to the computer and monitor is ON on the correct input (Some monitors have VGA,DVI, DisplayPort or HDMI, so its either automatic or you may need to select the correct input like on televisions)
Check if the cable is going either into the onboard ( blue port near where you plug in the mouse,keyboard,internet,printer,etc) or discrete video (usually bottom of the computer)
If the cable is not good, then simply change it.