When you get two dashes it means the motherboard is not starting the POST cycle. This can be caused by either a power, grounding, or a defective board. 

Make sure you have both the 24 pin ATX power and 8 pin power connected. Try booting the board outside of the case with only the 24 pin and 8 pin power from the power supply, the CPU and CPU cooler, and a single stick of memory installed on a wooden surface or piece of corrugated cardboard to eliminate a grounding issue.

If it still fails to POST, test the board with another power supply. If you still get two dashes most likely the board is defective and needs to be replaced.