getting Fatal error: Call to undefined function wp_get_current_user() on client site

a Day ago while working on a theme template I discovered that when attempting to update a plugin I now only get the following error when I attempt to go into the network admin areas of the system:
Fatal error: Call to undefined function wp_get_current_user() in /home/k7707128/public_html/wp-includes/capabilities.php on line 1356
I can only presume that either an updated item has added this call or it is in one of the documents I have added to the new theme I'm working on but the new theme does not have a call for wp_get_current_user() so where is this coming from it has locked me out of the administration area for the network.