Allison's HP desktop computer turns off all by itself. Now it won't turn on. Leo says to check the easy things first. Is it plugged in? Make sure the power cable hasn't come loose from the power supply. If all that is plugged in, chances are the power supply has died, preventing the power from getting to the motherboard. It's an easy fix, but it may require a proprietary power supply from HP. Even then, it shouldn't cost more than $100 to replace!