I'm currently updating the Bios. I literally had no idea I would have too. I hope it works! I've been working on this literally for 12 hours and tried every other solution and build another computer out of spare parts with the the same model MOBO to no avail.
Please note I UPDATED my above post ("M" versus non-M).
If there's no BIOS setting issue for GPU (probably not as you only have one now) it sounds like your CPU is either inserted incorrectly or is defective.
Since the PC already worked I don't know what else to suggest.
*I assume you still have the CPU 4-pin power cable hooked up.