My entire website other than the homepage, contact, and blog homepage spontaneously broke

I have no idea what happened, but my website, which was working beautifully appears to have spontaneously broken!

The most important broken page is the events page, as I have an event tomorrow and people can no longer sign up. I get the Fatal error: Class 'WP_Screen' not found in /home/content/p3pnexwpnas09_data03/24/2651924/html/wp-content/themes/upfront/elements/upfront-this-post/lib/this_post.php on line 539
when I click the link that previously worked.

The other pages do not work either. I'm not sure if this is due to a wordpress update (that I did not do) or some other issue...

Thanks for the help!