tried to update to 3.9.2 and site crashed

I followed instructions in an email I received from my host, GoDaddy, advising me to update my site to WordPress version 3.9.2 After doing so, I immediately got this message on the screen: Fatal error: Call to undefined function wp_get_current_user() in/home/content/80/9110680/html/wp-includes/capabilities.php on line 1329 and now I cannot login as admin nor can I view my website at all. Any advice on how to fix/undo update? Thank you.

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