Falls under "something unplugged," but to be sure. If you're going inside... Also make sure that all your fans are fully connected. See if the little fan for a graphics card has dust, or is spinning normally when you brush it (it should show a little bit of elastic springback at the end of moving from a tiny push, and not fly around without any bounce). If your system is really smart, some parts might refuse to operate when their fans are off. Some graphics cards simply can't.
A single piece of hardware in there can shut down the whole works if it's not happy. Fortunately, it's also cheaper to replace just one part.