jos-Error: Unable to load document class

JSite -> dispatch() @ D:\home\site\wwwroot\ap\index.php:42
JFactory :: getDocument() @ D:\home\site\wwwroot\ap\includes\application.php:155
JFactory :: createDocument() @ D:\home\site\wwwroot\ap\libraries\joomla\factory.php:192
JDocument :: getInstance() @ D:\home\site\wwwroot\ap\libraries\joomla\factory.php:864
JError :: raiseError() @ D:\home\:293
JError :: raise() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:251
JError :: throwError() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:176
call_user_func_array() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:214
JError :: handleCallback()
call_user_func() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:765
plgSystemRedirect :: handleError()
JError :: customErrorPage() @ D:\home\site\wwwroot\ap\plugins\system\redirect\redirect.php:109
JDocument :: getInstance() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:785
JError :: raiseError() @ D:\home\:293
JError :: raise() @ D:\home\site\wwwroot\ap\libraries\joomla\error\error.php:251