if user is logged in, show this, if not, show that… my simple code..

It will only display the “YOU’RE NOT LOGGED IN”
even if i’m logged in, could anyone tell me? or point me in the right direction?

<?php if ( $user_ID ) : // Check if logged in ?>
     YOUR BOOKMARKS 

<?php else : // User is not logged in ?>
     YOU'RE NOT LOGGED IN 

<?php endif; ?>

why isn’t this code working?