Your hard drive might be perfrectly ok. I had something similar happen to me just a couple days ago after an storm flickered the electricity off and on. I have my computer plugged into a power strip -- all I had to do was plug the computer into a different outlet.

The problem could be any one of serveral things -- most likely the computer is not getting any power. First, make sure it is plugged into an outlet that works! plug a lamp into the outlett just to test it.

If that is ok then the problem might be the power supply. But unfortunately most people don't have anything to test that.

If I were you I'd take the computer to a reputable computer shop, or to a friend that knows what he's doing and has the test equipment to work on it.

Does your computer display anything, or does it just display a black screen? Does the light come on? Is the CPU fan spinning?

I recently had to replace my motherboard after it shorted out. The screen did not display anything but the computer looked like it was running normally (The lights blinked and the fans spun). If that's the case, do like Ancient said and take it to a reputable computer repair store.
The data in your hard drive should be ok. If it is the motherboard, try to get the exact same model and brand that you already have, unless you really want to upgrade.
Changing hardware on the fly while using WinXP can cause bootup problems.