If there is still no activity, re-seat the memory modules and video card for they may not be properly seated and locked in place(some boards and cards have locking mechanisms that prevents them from unplugging themselves as the heat goes up when it is used due to thermal expansion). There are some motherboards that have issues with grounding; any contact with the casing (which is normally grounded to the earth prong of the power supply) would activate a circuit in the power supply that prevents it from working properly to avoid frying any sensitive equipment. Isolate the board by checking the stand-off and ensuring they are isolated from the casing.