This may not be a problem just an oversite. First make sure all the power cords and device cables are attached properly. Second make sure that any switches, possibly on the back of the computer, are turned on. Many common problems are due to loose connections either on the computer itself or at the power outlet.
My computer is on but I have no display.
Make sure the monitor is plugged in, connected to the computer and switched on. If your still have a problem restart your computer.