We are recently facing this problem out of sudden, wondering if there are any possibility why this problem happen.
Here's the error:
Warning: require_once(system/core/System.php): failed to open stream: No such file or directory in /var/www/html/index.php on line 32
Fatal error: require_once(): Failed opening required 'system/core/System.php' (include_path='.:/usr/share/php:/usr/share/pear') in /var/www/html/index.php on line 32
edit your "php.ini" file and change the value of "open_basedir" which allows the php to access this path. Then make sure your "include_path" points to where you want it to point (as suggested by people in the comments).