HTTP ERROR 500 you have any idea how fix such error???? first time on live server and

config.php

if ($_SERVER['SERVER_NAME'] == "localhost")
$config['base_url'] = 'http://localhost/repos/CodeIgniter-3.1.9_yyy_com_live/public_html/';
else
$config['base_url'] = 'http://www.yyy.com/';

/*
|--------------------------------------------------------------------------
| Index File
|--------------------------------------------------------------------------
|
| Typically this will be your index.php file, unless you've renamed it to
| something else. If you are using mod_rewrite to remove the page set this
| variable so that it is blank.
|
*/
$config['index_page'] = '';