I would unplug and reconnect all power supply connections. Especially paying special attention to those plugged into the graphics card and the monitor. Then, I would try again. As dumb as it sounds, I would also make sure that the monitor is plugged in and on as well.
Check the DVI/HDMI/VGA from the monitor to the graphics card or if you don't have one, the back of the computer, an dcheck that all cards are conncections from the power supply are plugged in properly. I would also replace the CR2032 battery on the motherboard while you are at it. If none of that works, message back and we can go from there.