wp-config.php has this in it before deactivation of the plugin:
<?php /** Enable W3 Total Cache */ define('WP_CACHE', true); // Added by W3 Total Cache /** * The base configurations of the WordPress.
And after I disable the plugin it looked like this:
<?php/** * The base configurations of the WordPress.
ie. the W3 Total Cache plugin forgot to put a carriage return back in the php file after it stripped out its contents and as a result the whole file was effectively commented!!
This kills WordPress. You need to manually add in the carriage return and then WP works again.