The power button is simply a "push to make" type, i.e. when you push the button the contacts make, when you release it the contacts break. You can achieve the same effect by momentarily shorting the motherboard power switch pins together. You can also temporarily use the reset button for the same purpose. When things don't work, I always like to revert to the minimum setup. Motherboard with CPU and heatsink, PSU connector, power switch and case speaker. This should boot to a series of error POST beeps because of the absence of memory or graphics card. If the motherboard doesn't respond correctly then you have very few components to investigate.
There is often a BIOS setting to start the computer as soon as power is applied.