If it were my problem, I would remove motherboard from case and put it on a non-conductive surface like a pile of newspaper. Then, I would put the build together (just 1 stick of RAM and CPU with heatsink) and see if it works outside of case. One time the issue was that I had not fully connected the cable to the PSU. Another time, I had not seated the CPU fully. Yet another time, I had 'lost a screw' behind the motherboard and it was shorting something. Another time, I had gotten the heatsink on wrong (so the CPU got hot and shut down quick). Looking back over the last 30 years of builds, I have messed up a lot. Hope this helps. Just go one step at a time through the installation instructions and it should sort out..
Just remembered another.. somehow a small metal clipping (probably during manufacturing) got down in a memory slot - took me forever to find it - had to turn mother board upside down and tap it on the desk before it came out.