I would do the following:
1) Rule out the monitor, by connecting it to a working PC.
2) when the computer boots up, does the power button light come on? Do you hear the fans running?
if so, the powersupply is good for now.
To rule out the harddrive. If the computer starts up and displays nothing at all and the fan doesn't spin it is likely motherboard/powersupply.
If it is solely your hard drive, you should be able to see the bios/post screens.
Test that for now.
Help!!!! My mother has a gateway that she bought about three years ago. She has had some upgrades done on it, however nothing lately. She called me the other day and said that her monitor wasn't working. I told her to bring the tower to me. I hooked it all up. And sure enough no display. But I don't think that video is the problem. When I turn it on I don't hear the harddrive start up. But also in order to get the computer to turn off you have to unplug the darn thing. The power button won't shut it off. What could possibly be the problem, Motherboard or harddrive. She has had the harddrive crash once before in it.