Computer Turn On But Monitor Stays Black Why?

My Computer Works fine and everything but after a while my computer just stoped working well nto really everything worked except when i turned it on I get a black screen. I know it' snot the monitor cause i hooked up my xbox 360 and it works and so does my secondary monitor. My computer will show it's busy cause that little orange light that shows when is doing soemthing lights up also my keyboard works but that's about it if anybody have any ideas i would like to hear them :0 thanks