What I would do is unplug the computer. Then pull out the 24 & 8 pin plugs. Then plug them back in.
Make sure your 24 pin plug is plug in all the way. Give that a try because the other day I was setting up a computer.
I didn't have the 24 pin plug in all the way. The board I was working the LED code display didn't light up.
I know that isn't the same thing but any little thing like that might cause the problem.