Similarly to loosing the user login page, it's not uncommon to remove the navigation links and blocks and suddenly you can't easily logout of a Drupal site. The answer to that is also to append a page callback to the address bar of your browser, either:
http://www.mysite.com/logout
or
http://www.mysite.com/?=logout
if you don't have clean URLs turned on.

Explanation:
Same as for the lost user login block, a 'page callback' invokes a function which logs the user out of the site(see Can't login to Drupal).