On several occaisions I have had the power go out on different PC boxes with Windows XP. When trying to reboot, the only thing that happens is the CPU cooling fan turns on high and stays there. Then, nothing happens. Powering down by pushing the off button 5 seconds and then restarting does the same thing. Then If the cord is disconnected from the power supply it will then boot as normal after the cord is plugged in and power button is pushed. I only learned how to get is going again by trial and error.