The motherboard could be shorting off the case or another component. This has happened to me a few times where for safety reasons the computer will not turn on at all. Repositioning the board in the case usually fixed the problem once I identified it.
Take the motherboard out of the case, next get a large static bag and some foam, lay the static bag on the foam and place the motherboard on the static bag. From there assemble your computer for testing purposes, just use the very basic components to see if it'll boot from outside the case. One stick of ram, GPU, CPU, PSU and the power switch from the case connected.