The possible reason as an overloaded power supply. Disconnect all external devices to see if any of those items are causing the problem, (such as a hard, diskette, or optical drives, and expansion cards). Power on the system. If the system enters the POST, then power off and replace one device at a time and repeat this procedure until failure occurs. Replace the device that is causing the failure. Continue adding devices one at a time to ensure all devices are functioning properly.