Fatal error on site and wp-admin

My homepage: Fatal error: Call to undefined function wp() in /home4/sdcart01/public_html/wp-blog-header.php on line 16

wp-admin page: Fatal error: Call to undefined function nocache_headers() in /home4/sdcart01/public_html/wp-admin/admin.php on line 33

I use Host Gator. I contacted them and they told me to contact my web developer; unfortunately that’s just me.

I have not made any recent changes, in fact I have not logged in for a long time, so I wonder if I missed a vital update. Unsure what to do now.

The page I need help with: [log in to see the link]