I had that happen recently. I turned it off while it was loading something, and it would not start back up. We had to end up reinstalling windows and everything else back on it. I was lucky and we were able to recover all my photos and music and such, but there is a chance that you won't be able too if you actually messed up the hard drive. I hope that is not the case. Here are a few links that might help assuming you are using windows xp. [link] [link]
I will try to find more links, but I could use some more info. My advice right now is to try to boot it up in safe mode (if you can), or try to reinstall window. You can do that by yourself, but I recommend taking it to a professional just to make sure that your data can be recovered, but there is no guarantee if it is the hard drive that is ruined. Hope I helped

--
9 out of 10 people agree that in a survey involving 10 people, 1 person will always disagree.
Teach me to draw [link] and take a look at my gallery while your there! [link]