I'd doulbe check all power connectors in the computer...perhaps something
came loose. Be sure to carefully check the connector...sometimes power
connectors look properly connected but are not. In particular, be sure the
motherboard power connector (usually a 20-pin or 24-pin plug) is properly
seated. Also, be sure the power to all the drives is properly seated. As
well, if your video card has a power connector, be sure that is fully
connected too.
If none of the above works, perhaps you may have a failing power supply.
How powerful is you power supply? what hard ware do you have installed (CPU,
motherboard, hard drives, RAM, etc...) ?