error Call to undefined function wp_get_current_user()

getting new error when ever I try to access updates from any sight in the network logged in as the supreme admin. I keep getting this error:Call to undefined function wp_get_current_user() in /home/k7707128/public_html/wp-includes/capabilities.php on line 1356

the line it refers to concerns use capabilities. the user I am should have ultimate capability so I’m more then a little concerned everything was working until 1:00am Monday morning. any advice would be welcome I currently have over thirty sites on this network and they must work. Or I don’t. thanks in advance.