My brother heard a sizzling sound, followed by a loud bang! He found his computer room filled with smoke, and had to open the windows. That sounded like a bad power supply to me.
I used iFixit's online procedure to remove the power supply, and sure enough, it had a blown-out capacitor. So I ordered a new power supply from iFixit, and had the computer working in no time! This confirmed my experience that iFixit is at the top of their field.
Buy a set of Torx screwdrivers from iFixit. I didn't realize that you need LONG ones until I started, and no one locally stocks them.