So, the computer just doesn't work? Just saying there's no video on the monitor doesn't mean it has to have to do with the videocard, it could be any number of things from powersupply, cpu, motherboard, ram, shorting problem, BIOS issue, etc.
Did it stop working, or was it not working to begin with?
To start off, first try to clear the CMOS with the jumper near the battery.
If that doesn't help, next thing to do is to disconnect everything you don't need to boot up (keep motherboard, powersupply, cpu, ram, videocard) and then try to boot up.
If still nothing, take those components out of the case and place the motherboard on a piece of cardboard. Connect the components listed above. Short out the pins that the power button was connected to.
If it still doesn't turn on, then there's probably something physically wrong with one of the basic parts of the system preventing it from booting.