Will Not boot!!!

I just purchased new power supply and motherboard from tigerdirect. Installed it with an old cpu. When I turn it on the led on the motherboard comes on but the fan on the cpu doesn’t spin and nothing powers up.tried unplugging all drives. Even tried running the motherboard outside of the case to make sure it wasn’t a ground issue. I don’t think its the motherboard or psu. Could it be the cpu?