Warning: Parameter 1 to modMainMenuHelper::buildXML() expected to be a reference.

Solution :

This error is  occurred due the php version 5.3 and joomla is not certified to work with PHP 5.3.0.

You need to change some code in your files.

open the file
vi /home/username/public/modules/mod_mainmenu/helper.php

You can change this :
Form

function buildXML(&$params)

to

function buildXML($params)

Save the file try to access your site.