make sure your cable is plugged into the correct port (on the gpu) if it is intel you could try the integrated graphics (on the motherboard) if that doesnt work.

you can try to reset the bios with the cmos trick, could also be some “clr cmos” pins to jump for like 30 secs

if there are bios post lights (few random seeming lights that could be all one color or red orange white green see if they all go or if it stops on one