As Mary suggests a screen shot would help confirm my thoughts here but I think you are talking about the 'Activities' block on the left-hand side of the page...
The Activities block simply lists all the different types of activities used within the course and provide a shortcut to that activity's list page (eg http://moodle.org/mod/forum/index.php is the Forum list for this course).
What you have done (if I'm guessing correctly!!) is removed all assignments from the course, because there are no assignments to list for the course the 'Assignment' link is not displayed.
I hope this makes sense and is helpful
Cheers
Teresa