Was it working before this problem began? Did you add or remove anything, before the trouble began?
did you download anything before this began?
I would remove the power supply connectors, except for the 20/24 connection. the RAM,the connections for your drives, mouse and keyboard. check to see if the frontal panel connections are correctly connected, replace the battery, and if your video card is not integrated, remove it.
then plug in your mouse, the PSU connector to the CPU, and the connector for your hard drive. Turn the pc on. If it works, then add one of your RAM Bars, check again. ADD another RAM bar, and check again. then add another connector, reconnect your video card(if not integrated),check again. keep adding one connection...