Hmm, you have already started the fault finding process-- power to the hardrive and the cpu fan spins, so we can discount the psu.
Is the moniter powered via the pc or from the mains? is the power light on the moniter on? if not check the fuse.
I am certain it is not to do with graphics cards, as you mention no beep on startup. also unlikely to the mainboard gone. Have you been "fiddling" inside the case and dislodged a lead etc??
Have you changed the memory and not seated it correctly?
Try the above and post back