Computer won't shut down

Im new here so sorry if I do anything wrong. Recently I bought all the parts of a computer individually and put it together and plugged in a monitor. When I turned on the computer nothing happened on the monitor and when I tried to shut down the computer by holding down the power button nothing would happen. The only way to shut down the computer was turning off the power supply or unplugging the power supply. Can anyone tell me what is wrong? Nothing happens on the monitor, theres is no operating system installed because nothing happens to the monitor so I can't do anything, I sent back the motherboard and cpu and got a new one but they could be defective again so I am completely lost and have no idea what is wrong
 
Okay, did you:
use standoffs under the motherboard only where there are mounting holes in the motherboard... usually 6 and no more?
use thermal paste between the CPU and it's heat sink?
make sure all of the power supply cables were properly connected to the motherboard and the socket near the CPU and on the video card?
make sure that the motherboards bios was cleared by using a jumper or removing a battery?
 
Lol uh what are standoffs, yes to the thermal paste I think is it the sticky thing under the fan?, im pretty sure all cables are correct and how do I do that last part?
 
Standoffs separate the motherboard from the case bottom, except where the mounting screws go
 
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