DELL R750XS, Won't Boot

So one day we find our dell R750XS is off and all our servers are running on UPS. All other servers were on but the R750XS.

At first, all there was is the blue (i) status button, pressing the power button did nothing. No spinning FAN but just a single green led on the motherboard.

Tried re-seating RAM, CPU removed all cards, HDDs, still nothing.

Stripped it down to minimal boot requirements, 1 CPU, 1 RAM on slot A1 etc, still no response.

Later tried removing all cables on the motherboard and putting them back. This time a few more lights came on on the MB. Once power button is pressed, it glows in and out a green color. Once left for maybe 30 minutes, the Server will come on, all fans spinning, power supplies are green, but it won't post. The screen remains blank (no signal). Tried both VGA ports but still the same response.

iDRAC port is responsive but will be no use as we unfortunately never enabled it. Stuck there.

Could it be faulty motherboard, CPU? Please Help