I have a problem with a PC which had been working fine, but suddenly won't boot - not even a message on the monitor which remains completely blank (and yes the monitor is fine!). It checks the floppy disc drive, and that's as far as it gets. Repeatedly accessing the floppy drive as though looking for a disc, though when I tried putting in a boot disc this was not read. I tried a new floppy drive but the same thing happened. The PC just keeps on starting the boot sequence but gets no further than checking the floppy drive.
It should at least show the motherboard screen, if it doesn't then I would be inclined to check the monitor first and then the graphics card. Make sure your internal speaker is connected cos as Indigo 1 has mentioned this can actually tell you what it is thats wrong.