Urgent help needed to restore Wordpress

Our multisite network has gone down completely without warning.

We are getting this message:

Warning: implode(): Invalid arguments passed in /home/revolutionarts/public_html/wp-content/plugins/wp-hummingbird/core/modules/minify/class-minify-group.php on line 692

Warning: implode(): Invalid arguments passed in /home/revolutionarts/public_html/wp-content/plugins/wp-hummingbird/core/modules/minify/class-minify-group.php on line 695

Fatal error: Call to a member function get_page_permastruct() on a non-object in /home/revolutionarts/public_html/wp-includes/link-template.php on line 357

Any advice on what we can do to restore it asap?

Thanks
Michael