Occasionally, Joomla will have a small hiccup and you can experience 'strange' errors such as the following: Fatal error: Class 'JConfig' not found in ......../includes/framework.php on line 56
Not to worry, a fresh Joomla core install will fix this.
These are the steps to fix this problem:
1. First and foremost create a backup of the web server and database server on your remote host.
2. Download a fresh copy of the Joomla Stable Package
3. Unzip this on your local pc using 7-Zip or similar
4. Delete the installation/ directory (absolute priority)
5. Upload the files to the same directory on your host where these files already exist and overwrite them, using an FTP client WinSCP or FileZilla.
6. Check the functionality of your site.
:: I found these directions here: http://forum.joomla.org/viewtopic.php?f=615&t=723527