Sites not served after Precise - Trusty upgrade - new apache2 documentroot path
An existing site is no longer online following a Precise to Trusty upgrade when the site is in the default DocumentRoot. This is because DocumentRoot on Trusty is now /var/www/html whereas it was /var/www for Precise.
There is a simple work-around for simple deployments: move the contents of /var/www to /var/www/html. More complex scenarios may require additional reconfiguration, such as anything else referencing these as absolute paths.