The power supply converts the 120 volt AC power coming from the wall into very tightly regulated DC power that the computer needs to operate. If the computer is subjected to a power surge, the power supply takes the brunt of it and is usually the first component to fail. Many power supplies are very cheaply made and will fail without any apparent cause. If your computer doesn’t show any signs of life at all when you try to start it, odds are the power supply is dead.