Dell Inspiron 6400 won't turn on

Question

I left the laptop in sleep mode in a carry case (with half empty battery) after unplugging the charger for 5-6 hours. It then won't turn on so I thought it was a flat battery. After fully charge the battery it still won't turn on with either battery only, AC only or both sources. The Numlock, Capslock and Scrolllock LEDs turn briefly on for a couple of seconds then off when the Power button is pressed. There is no screen activity, no sound of anything moving inside the laptop. However the charging circuit seems working as normal.