Lost website

Hi can anyone help I had a website https://www.winchesterbouncycastles.co.uk and I did an update(or thought I did) and now all I get is this: Fatal error: Call to undefined function wp() in /home/winchest/public_html/wp-blog-header.php on line 14

I have no idea what it means but would love to get the site back.
Thanks
Steve