Ah, so the part where the keyboard goes back in. It was a plastic piece? Sometimes they are metal. If it is the metal type, that may be able to be replaced. Otherwise, you probably need a new board, although I am not sure. I would plug in the keyboard and just put a piece of tape over it to hold it down.
EDIT- I just looked at a pic of that. I have the same computer, or an extremely similar one sitting at home. I have taken it apart, and I know what you are talking about. I don't believe there is a way to replace it without a new mobo. For that computer, I am not sure how much it would cost. However, I would try to tape it and see how that works for you, as most laptop mobo's run around 100-200 bucks, and if a tiny piece of tape would work, that would be better than spending all of that money.
Good Luck!