1. The CMOS reset jumper is... jumped.
2. The motherboard has a short to the case with some extra standoffs or in some cases the owner mounted it with no standoffs.
3. The PSU is dead on arrival. Unplug it from the motherboard and perform THE GREEN WIRE PSU TEST (google that.)
4. A common mistake new builders make is to put all the parts together in one go. Nope. Start small so you know when the problem began.