Getting error Warning: Parameter 1 to modMainMenuHelper::buildXML() expected to be a reference, value given in /libraries/joomla/cache/handler/callback.php on line 99

This error happens because the version of Joomla! you are running is not compatible with PHP 5.3.0 . You could get this error for the reason being that your host upgraded to PHP 5.3.0

A quick solution will be is to open the file modules/mod_mainmenu/helper.php in your root folder and change the following line from

function buildXML(&$params)

to

function buildXML($params)

at the top of helper.php

The best solution will be to upgrade your Joomla! installation to the latest Joomla! release