This happens on my laptop - it's usually spilt coffee or a crumb from a sandwich fouling the key cap.
Until you solve it you might consider using either the Character Map or the On Screen Keyboard - Help will tell you how.
Here's a link to a web page that shows how to repair a broken laptop key - just pull your poorly key cap off and give it a good clean with an old toothbrush then reassemble as per the link below...
http://www.laptoprepair101.com/laptop/2007/03/20/key-fell-off-keyboard/
Good luck...
Peccavi, September 2009
