Hello and welcome to PC Help Forum.
I would check all your connections.I just built myne and powered on brilliant.
Have to attached 20 or 24 pin power supply?
Are you sure it is all connected correctly?
Are you sure power on button etc is connected in right order?
Did you use new
RAM/Memory?
What are the specifications of your new build?
Are the fans connnected to mother board or main power supply?
Could you post any clear pictures of inside your pc?As pictures would give us a clear knowledge of how it is setup?
EDIT:sorry Gandalf didnt see you there.