Need a bit more information, an FX-8120 with just the motherboard should power on fine as it uses much less than 500 watts. Are we missing something like the video card in the equation and other peripherals? In addition it could also be that you installed the front panel connectors from the computer case the wrong way if this is a new build. It could also be if you have not connected the 4/8 pin power connector for the CPU. Lastly, you could also check if the bios version supports the CPU.
Edit: Just checked its an 8 pin connector for your motherboard.