Do you have the keyboard plugged into the motherboard USB or keyboard slot?

Do you have a spare keyboard you could test with? it'd be prudent to make sure that the keyboard itself works at this point, and isn't malfunctioning in some way. 
 
You may also consider pulling all but one stick of RAM out of the board. Make sure that one stick is in the 1st gray slot, 2nd innermost from the left. If that fails to work, cycle through them until it either works, or none of the sticks allow the system to post.
 
Also, if your motherboard is installed in a case, try the setup in a barebones configuration: Pull the board out of the case, install one stick of ram (as before), no HDD/SSD, one video card, and just your keyboard as a USB device. Make sure the board is sitting flat on a piece of cardboard or non-conductive material on a flat surface. Then, attach your power supply and try it again. If it works, that means there's a short of some kind between your motherboard and case. Make sure your IO backplate is installed correctly, and nothing is making improper contact. Also, make sure the brass case standoffs are only in spots where there is a mounting screw hole on the board.
 
Let us know how that testing goes.