I had that error when I deleted accidentally some code from wp-config.php
Check if you got the end of the file:
/* That's all, stop editing! Happy blogging. */
/** WordPress absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
define('ABSPATH', dirname(__FILE__) . '/');
/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');
?>
That worked for me.