Check the case, something could have jarred loose also, video card maybe.
Another thing to check, some systems have issues with the motherboard capacitors (very common in some models, rere in others) that would cause odd issues such as working one day, not the next day. Look on the motherboard for any puffy or outright leaking capacitors. Even a little bit of puffiness is not good, they should all be flat (look at ones with X on the top especially).