Unable To See Admin Dashboard, How to Manually change the admin theme
This is the easiest method to reset your admin theme in a Cpanel. 

Update your settings.php to have:

// If your admin theme is Bartik.
$conf['admin_theme'] = 'bartik';


Warning with using $conf as you cannot set the theme via Appearance page anymore.

If you prefer SQL query method...


Access PHPMyAdmin.
Select the Drupal database and run this query.


UPDATE variable SET value='s:7:"garland"' WHERE name = 'admin_theme';

Where 7 is the total number of strings your theme has. Example, if your theme is adminimal. Then your query should be like this.

UPDATE variable SET value='s:9:"adminimal"' WHERE name = 'admin_theme';


Please see the documentation on Drupal.org.

Take note that if you are using Context module the $conf is not helpful as it will always respect the theme assigned via Context.

Hope this helps.