When you turn the computer on, does it beep? If not then chances are it's not POSTing which is a sign that something is wrong with the HW installed. Did you install the hardware yourself? Maybe you put a little too much pressure on the core of the CPU when installing the heat sink. Maybe the CPU and/or motherboard were defective when purchased. Maybe the RAM is dead. There's a lot of possibilities.

Like Interman said, it COULD be that the video card isn't it properly, but usually it'd beep a lot if that was the case. Oh and all the things he outlined are valid too, of course.