You said if you plug the cable into the motherboard (you called it vertical) your monitor displays fine and boots win too, doesn't it? If so, try to boot pc with onboard gpu (with your pci card plugged out) , then go to the driver app and change the overclock settings to default, assuming you overclocked it this way. If sth cant be done try with plugged gpu to the motherboard. Then change the cable position (into your pci gpu card and see if you can boot it.
This problem occurs too if your ram doesnt work properly or is plugged in incorrectly. Maybe you have sth with that.