[Lynne]

Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true); 


Also, look to see if anything is in your error_logs (if you don't know where they are, ask your host).

And, let's see what is in Maintenance > Diagnostics > Problematic Webserver Modules