Next, check if the power supply has a switch at the rear if it is on (older models) and see if it works. Check for any foreign metal objects that may be stuck between the board and the case which is shorting it out, again the protection circuit would prevent power up to avoid damage to the various sensitive electronics. A faulty motherboard fan can also be causing the problem so check the fan if it works. You might have also forgotten to connect the power button to the board (along with the reset button) so check that too. A shorted reset switch can also keep the board in a state of permanent off, unplug it to see if that works.