xenforo too many connections

I got this error 4 times recently: stack Trace #0 /library/Zend/Db/Adapter/Abstract.php(315): Zend_Db_Adapter_Mysqli->_connect() #1 /library/XenForo/Application.php(727): Zend_Db_Adapter_Abstract->getConnection() #2 [internal function]: XenForo_Application->loadDb(Object(Zend_Config))...