Possibly check and make sure that your moniter cable is plugged into your video card all the way. I know this happened to me before.
You can also test your moniter on another CPU system, perhaps it's a moniter issue.
I've had this problem when you turn on the pc the monitor stays black and the pc is on. Last time I fixed it by making sure the ram was in all the way but it is right now. Its not power supply because its new. When I turn on the pc there are two lights, one with a battery under and the other with something that looks like a bulb. The batter light is steady red but the other light is flashing yellow. I dont recall seeing this before. Any help on this?