Seems like someone turned off the display of the define page either in your custom template or the default Zen Cart template by modifying the code. In this case, nothing you do from your Admin will be of help until you make changes to the affected files ...
Check the following files:- includes/templates/theme028/templates/templates/tpl_index_default.php
- includes/templates/template_default/templates/tpl_index_default.php
and make sure that you uncomment the following line.
PHP Code:
<!--
<div id="indexDefaultMainContent" class="content"><?php require($define_page); ?></div>
-->
Try it and let us know.