Off the top, two suggestions:
First, make sure the power cable from the power supply is plugged in to the wall socket (obviously). Then trace the PWR cable running from the front panel power switch to the mainboard, Unplug that cable at the mainboard. Short those two contacts with a screwdriver and the computer should power up and you get the POST screen on the monitor. If you short those leads and nothing happens, the problem is:
1) dead power supply...also, make sure the on/off switch on the back of the power supply (if it has one) is turned on.
2) power supply main power cable is not plugged into the mainboard
3) possible short on the bottom of the mainboard touching case metal
4) bad mainboard
The ultimate test is a bench build, which is what I always do. Here is a link to the steps I take, if you are interested.
Wish you the best, I know it's a bummer when your newbuild has problems. Let us know how it goes!
Edited by ranchhand_, 05 April 2018 - 01:23 PM.