"unplug the monitor lead and plug it back in" Done it no joy
"Does the machine boot up? Can you see flashing lights and hear the hard drive working? If not that is your problem". All looks and sounds OK
Open the case and see if the cooling fan on the CPU is spinning. If yes then your machine should be ok. Your graphics card may have come loose - try pushing it back down or taking it out and then plugging it back in.
Will try this later when i get back Edit just done this no joy I assume the graphic card is the one the monitor plugs into
If the board is dead then I suspect a power surge - a new power supply should do the trick. Sorry what board? all power looks ok
Remember It is only possible to live happy-ever-after on a day-to-day basis