MyBB has experienced an internal SQL error and cannot continue.

[Admin]

SQL Error:2002 - No connection could be made because the target machine actively refused it.

Query:[READ] Unable to connect to MySQL server

Please contact the MyBB Group for technical support.

edit inc/config.php 

$config['database']['hostname'] = 'localhost'; 

to

$config['database']['hostname'] = 'localhost:0000';

usually in this case scenario it's because your MySQL or Mara is using a different port other then the default 3306