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.