Booting / Staring Problems for my PC

Hi, I have a P4 3.04 GHz, with 2 GB Ram and 500 GB HDD (Sata2). From Yesterday, I cannot boot / start my PC. When switched “on” only I see power LED to turn green, other than this activity there is no activity in my PC. No starting of fans, No Humming from HDDs. Absolutely No activity. Even my monitor does not register any thing. Even POST activity is not observed.

Kindly help me in identifying what went wrong ?