Fatal Error when using a link

So I’ll be brief, I was given access to our company website to help with admin duties and keeping things updated. Our owner had a falling out with the creator of the site and I am a novice at best.

I am getting this error when clicking on a link from the homepage:

Fatal error: Call to a member function is_singular() on a non-object in /home4/getfit/public_html/wp-includes/query.php on line 618

and ideas on how to fix this?