unable to view site, unablet to login

Hello,

I am having a problem with my wordpress site that is preventing access to the site, and unable to login.

when I try to load the homepage https://www.abruzzischoolgarden.com I get a blank page, when I attempt access, (wp-login.php is loading) I get the following messsage:

Fatal error: Call to undefined function get_blogs_of_user() in /home2/abruzzis/public_html/wp-includes/class-wp-admin-bar.php on line 18

I would greatly appreciate your assistance in this urgent matter. I work for a non-profit, and our donors need to be able to access our site this week.

Thank you for your time