I am also having the same problem.
The CX750 is a standard PSU. In what way does it "not work" with the mainboard?
I would think so too, but for a certain reason this motherboard seems to have compatibility issues with some power supplies, and I can't wrap my head around it. I first bought this MB with a Seasonic Platinum 860W power supply. With it, the computer POSTed fine for the first boot and I thought everything was A-OK, but on the second boot the computer wouldn't display or do anything. The internal motherboard light, however, remained on.
I googled for an answer and all I found out was that a computer not POSTing was 'either a motherboard or power supply issue'. I borrowed my friend's Cooler Master 500W power supply to test this, and on the first boot I got a 'fixing corrupted bios' message, everything working fine then on. I assumed it was all a PSU issue and exchanged it a week later.
What I ended up exchanging the Seasonic for is a Corsair AX860i power supply. Again I installed it into my computer replacing my friend's Cooler Master and everything worked perfectly... the first time around. I left the computer on overnight and when I came back to it, it had shut off. This was predictable behaviour, as I thought the computer had simply gone into sleep mode. However, it became apparent that the entire thing had turned off, and again I could no longer turn it on for more than a fraction of a second (slightly better than the Seasonic that gave no indication of anything turning 'on' at all).
None of the wires had been loosened or moved in any way (although I did double-check them to make sure they're fitted in properly) and I was getting the exact same problem as before. Once again the internal motherboard light remained on with nothing else working. Now Seasonic and Corsair are both very reputable companies for their power supplies, and the chance that each of their top-tier PSUs failed in a few days is pretty much nil. Additionally, the Corsair has a self-test button that passes with the PSU turning on when it's UNPLUGGED from the motherboard.
This fiasco is driving me nuts. The Cooler Master 500W worked perfectly for the week that I was using it, while Seasonic's and Corsair's 860W power supplies seem to only work for the first boot or so before failing for no reason. While I didn't test the Seasonic afterwards, the Corsair's self-test suggests that the power supplies DO remain functional and are not damaged by the motherboard. It should also be mentioned that while I was using the Cooler Master power supply that did work, I upgraded the Gigabyte motherboard firmware to F11 (latest). Also, a reviewer
here describes the exact same problem, but there is no mention of it anywhere else. My best guess is that 'large' wattage PSUs are corrupting the MoBo's BIOS after the first boot, but I'm still dumbfounded by the idea.