|Check my # 2 here:|
Some of it may apply to your situation and some may not. You obviously messed up something while you were inside the case. You may have damaged a component or maybe just jostled something loose. If you have a video card instead of on-board video, make sure it is firmly seated. Even if it's a little loose it may not work.
You may be able to isolate the problem by disconnecting all the devices and then reconnecting them one-by-one as I mentioned there.
Obama's a 2012'er. That explains EVERYTHING..