How do you know what is working if you can't see anything? Just because it makes sounds when you turn it on does not mean it's working.
First, try a new monitor, if that does not work, check over the motherboard for anything busted, like capacitors that are puffy or leaking. If that looks good, try a new video card. If that does not work, try a new power supply.
What are you system specs? I would reseat and unplug and plug back in the power connections to the graphics card (if you have one). Then check the display cable to to the monitor and make sure it hasn't become lose.