Undefined Function after updating

I decided to reactivate my blog today after letting it sit fallow for around a year. I updated WordPress to the latest version, and ran updates for all of my plug-ins. After the WordPress update, I can’t access my dashboard. The blog is there, and people can read it like normal, but when I try to log into my dashboard I get the following error –

Fatal error: Call to undefined function wp_json_encode() in /homepages/36/d195551902/htdocs/blog/wp-admin/includes/misc.php on line 668