what about ur connection to the monitor u know the video cable, is that working or not? Can you hear the hard drive whirring and all the fans running when u switch it on. Also do put ur PC config on here as well mate for us to have a bit more information to work on.
so if everything is working that does kinda rules out the PSU issue then cos that would have meant that the fans wudnt work and the other stuff on the PC wudnt work either. Have u tried switchig it on without ur graphics card. May be its a bad grafix card thats causing the problem. BTW does it blink or any other sign on monitor indicating that the PC is alive and switched on???
Most modern machne have a cpu test and reort by voice if the cpu is not working! (that suprised me too!)
However I have had exactly your problem with a clients recently and the reason was the video card fan was blocked with dust and stopped rotating (all the rest appeared to work) and by cleaning the fan(s) the heat on the video card was reduced to a level where it all worked again. heat can be a real problem even on desktops hope it helps!