Are all your cords connected? Check that first. As for the power supply, go ahead and replace it, with an identical one if at all possible. Don't EVER, I repeat DO NOT EVER open your power supply because it contains capacitors that will hold a charge for long periods of time. You could die from an electric shock. Anyways, after checking the connections of your cords, using an electrostatic ground strap, check the connections of your memory (RAM). Take them out, blow off the dust, and replace them, making sure they are seated well.