If it's a laptop and if you're anything like me it will be debris under the key cap. This site shows you how to fix it - I use an old soft toothbrush to clean the crumbs away...
http://www.laptoprepair101.com/laptop/2007/03/20/key-fell-off-keyboard/comment-page-11/
In the meantime you can find the @ symbol in the Character Map
Start / Programs / Accessories / System Tools / Caracter Map
Peccavi, February 2009
