Fix white screen on a Joomla site

I managed to activate errors on the site and fixed a few, but now I'm stuck and do not know how to proceed. The administration section is working fine without any problem. Disabled and enabled plugins without any joy. Only the frontend showing the white screen. Site is currently running Joomla 2.5.14. The error I now see is: "Fatal error: Call to undefined method JSite::execute() in /public_html/[login to view URL] on line 45" Line 41 to 45 of [login to view URL]: "// Instantiate the application. $app = JFactory::getApplication('site'); // Execute the application. $app->execute();" The code stops at line 45. Can anyone please assist to get the site up and running again and what will it cost? Look forward to your urgent response.