After updating Joomla to 3.1.4 version your site may give 500 error or display the following error message that relates to EF3 Framework plugin for Joomla 3.x
PHP Fatal error: Class 'JFile' not found in .../plugins/system/jmframework/jmframework.php
All you need to do is just updating the EF3 framework to make the template work correctly.
You will find the latest version of EF3 framework in your download area.
We have already fixed the issue in EF3 framework by replacing:
/plugins/system/jmframework/jmframework.php on line 146
But if 500 error still appears on your site it may relate to extensions installed on your site.
Joomla! Project team released hotfix that you can install in the standard way and solve the problem that will be fixed in Joomla 3.1.15