Actually number of beeps and their length can easily mark the problem. Look for it in your motherboard book, maybe you will find the answer there. Check also if you have beeper (or speaker) plugged in your motherboard (bottom right). You can also try turning on PC without GPU to exclude the GPU problem.
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.
I've already done it and it works in my first try with safe mode. But when I shut down my computer and then monitor didn't display I've tried my montior plugged it into another laptop and then it display in laptop.
Btw is it right:
Plug my monitor to another comp/laptop, then restart it in safe mode, after that unplug it and then plug it into my computer. And it still didn't work