If you suddenly start to get the error “Fatal error: Call to undefined function get_option() in” “/wp-admin/admin.php” or “Fatal error: Call to undefined function wp() in” “/wp-blog-header.php on line %%” when trying to access the admin of your WordPress website chances are your WordPress installation has got corrupt.
Geneally it could be a few files that need replacing but its a good idea to replace the entire WordPress Core installation.
At this point you will need to identify the WordPress version installed and then download the same version from https://wordpress.org/download/release-archive/ then just overwrite the existing installation files with the contents of the file you download.
To find the version of WordPress installed you can generally go to the front page of your site(which should be working) and view the source. Then search and find the line that looks like
<meta name="generator" content="WordPress 4.0.4" />
In this example the version is 4.0.4