Update Network not working

Ever since upgrading I get a message saying “Thank you for Updating! Please visit the Update Network page to update all your sites.” I also get a message saying I have a Plugin update. If I click on either of these, or if I click on My Sites/Network Admin, I immediately get an errorr message as follows:

Fatal error: Call to undefined function wp_get_current_user() in /home/rcastell/public_html/wordpress/wp-includes/capabilities.php on line 1187

I’m rcastell

I’ve searched WordPress using ftp and I can’t find a capabilities.php file, and I would know what to do with it if I did.

My current site (the only one) is working fine and I don’t want to disrupt it because its about to go live, but I was planning to show some colleagues how easy a WordPress.org site is to set up!

I’d appreciate any advice.