Call to undefined function get_header() in 404.php

You could do to your theme template files like several core admin files do at the very beginning of the file, placing code like this:

// don't load directly
if ( !defined('ABSPATH') )
	die('Cheatin’ uh?');

This will prevent subsequent PHP errors when the file is requested directly. The constant ABSPATH is defined in wp-config.php, which is called for every valid WP request. Not being defined is a sure sign that there was an attempt to bypass loading WP, be it on purpose or by accident. It’ll take a lot less than 3 day, even if you’re not a developer. To be sure, it’s only a stop gap measure, and copying template files to a child theme just for this purpose is questionable.