If you mean a key cap no longer works...
http://www.laptoprepair101.com/laptop/2007/03/20/key-fell-off-keyboard/
will show you how to fix it... pull off the naughty key - give it a clean with an old toothbrush - reassemble as per the link above.
My keyboard fill up with crumbs and drips - it's a regular thing if you graze at the computer...
Good luck
Peccavi, November 2009
