What makes a maintenance mode message display, when a site is not in maintenance mode?
If not in Maintenance it could also be an uncaught PHP exception as suggested here: &quot;The website encountered an unexpected error. Please try again later.&quot; so check your Drupal logs (Reports &gt; Recent log messages), or a problem with your database service. 

You can test whether it's a database offline situation by using a special theme template based off of maintenance-page.tpl.php called maintenance-page--offline.tpl.php where you put your own HTML and text instead of printing the $content variable: 

&lt;h2&gt;Connection to database lost!&lt;/h2&gt;
&lt;p&gt;[Suggestions about what to do...]&lt;/p&gt;


You can test what the page looks like (on your local dev environment) by disabling the mysqld temporarily and visiting your local site. Don't forget to clear the cache to get the new template to register before trying to test it.