take everything out of the tower.. and put your motherboard lay down on the box it came with and try to power it up with bare minimum, motherboard cpu cooler and nothing else
![Stick Out Tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
if it powers start adding components if everything works, that it means the motherboard was shorting on the tower somewhere (usually in the back between mobo and tower), now in the case that it still doesn't work make sure your power supply unit is working(and connected to the wall) and if u test it on another pc and it works fine then it means the motherboard is faulty.
(make double sure you connected the power button cable correctly on the motherboard)