If WP-CLI is properly installed but is not returning output, it could be any number of reasons. Check to see if your server is using Suhosin. You may have to edit the php.ini file to whitelist. Also check to see if your site is hacked by a web shell. If the command wp –info is working but no other commands work, check your wp-config.php file. If there is injected code in this file, it can break WP-CLI.