Am I understanding correctly that nothing is happening (aside from maybe fans spinning) when you power on the PC?
Assuming everything is plugged in correctly and you've checked and rechecked all your connections, I'd take the entire thing out of the case, reconnect everything outside of the case and see if you can power it on,
just like this.
The screws you have connecting your motherboard to your case don't appear to be insulated, which could be causing a short and thus the behavior you're seeing here. I've seen this many times before, most recently on a computer a friend built that exhibited the same symptoms you're describing (Power, fans spin,but no display or the appearance of any other life).
Once you've got it out of the case, you can carefully jump the two power pins with a flathead screwdriver to get the machine to start.