It's unlikely to be the hard drive - but if it is, the system will power up if you remove the drive;  try that.

Verify that the LED on the AC adapter is ON -- if it's not, replace the adapter.

If it still won't power up:

Remove the battery and try.  Power up = battery bad;  replace.  No power up:

Reseat the memory module(s) -- remove and replace.  Still nothing?

Bad mainboard.

It powers on but nothing else, all the lights on the computer flash like normal startup, then the screen makes a quick blink, and then nothing, the power light stays on.