Err... if you don't even know what's missing, it can't be something you were using? Unless you accidentally uninstalled the entire application it should still be in your Start menu somewhere.
As far as how it happened... you probably unpinned it accidentally. If you right-click an icon, the "unpin" action is the one at the bottom of the menu, closest to the mouse pointer. Not hard to hit that by accident, and it doesn't ask you to confirm the removal. (I'm assuming Windows 7 here.)