JError: Unable to load Database Driver: mysql

jos-Error: Unable to load Database Driver: mysql

JSession -> __destruct()
JSession -> close() @ D:\home\site\wwwroot\ap\libraries\joomla\session\session.php:133
session_write_close() @ D:\home\site\wwwroot\ap\libraries\joomla\session\session.php:676
JSessionStorageDatabase -> write()
JFactory :: getDbo() @ D:\home\site\wwwroot\ap\libraries\joomla\session\storage\database.php:71
JFactory :: createDbo() @ D:\home\site\wwwroot\ap\libraries\joomla\factory.php:308
JDatabase :: getInstance() @ D:\home\site\wwwroot\ap\libraries\joomla\factory.php:710
JError :: raiseError() @ :295
JError :: raise() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:251