You must turn on PHP error reporting. To do this, you should change the setting in your php.ini config file. Temporarily insert the following lines in your index.php file:
error_reporting(E_ALL); ini_set('display_errors', TRUE); ini_set('display_startup_errors', TRUE);
Make sure that you have inserted the code directly after the opening PHP tag. The errors should now appear directly on your screen. Alternatively, check your log files for the most recent errors.