Totaling 86 comments this troubleshooting post came back with the motherboard probably being the culprit.
Later on we have a new motherboard and the system still does not work. More specifically, all fans power on and stay powered, just not video output. We have tried iGPU and GPU. We have tried multiple monitors and cables. We are building it out of the case, barebones. We've tried SIMM and DIMM RAM configs.
We don't have access to compatible RAM, CPU and it can't be the PSU because the fans work.
That's not necessarily true. Years ago I had a build for my brother that even though the fans still worked and everything lit up like it was supposed to, both the PCIe slots and the iGPU failed to output any video. Was terribly dumb and ended up frying another system with that same PSU when trying to troubleshoot.
As @Aezen said that isn't necessarily true. I had a cheap PSU in my first build that was supposed to be "1000W" but was really "800W" and was probably more like 12. Consequently it would randomly hard reset at random intervals. I finally replaced it and never had another problem.
I had a customer's PSU go with similar symptoms. The only difference was that his molex powered fans would turn on and off, but the CPU fan would run consistently. Seems like PSU.
could it be software? could the cpu not be supported by that specific motherboard without a bios update? i saw earlier you update the bios thinking it was borked, btu did you update with the newest one? what cpu? is it a xeon?
if it isnt a bios issuse it's probably power. i have a old apu build and one day i forgot to plug in the 4pin, but i did plud in the motherboard power and the fans run and the lighs turned on but it wouldnt post as the graphics didnt have enough juice without the 4pin. it sounds like your situation, not enough power. what gpu? how bigs the psu? water cooling? ect....
however i would look into the software first as fixing it would be cheaper then buying a new psu only to find that the problem was the bios vrs.
He went down to the local PC store but the guy didn't have any spare compatible CPUs for him to try. Without buying, I guess. I'll hear from my mate again early next week.