Script-loader.php Critical Error

A client’s website has gone down. I’m not sure if someone has updated something in my absence, or if maybe WordPress has updated itself and caused a problem (that seems the case). In any case, I don’t have a back up from before the update to roll back to, and wonder if anyone has been able to fix the 2850 line error and how? I’ve been searching for how to fix it, but can’t find anything that works so far.

Fatal error: Cannot redeclare wp_add_iframed_editor_assets_html() (previously declared in …/wp-includes/deprecated.php:4312) in …/wp-includes/script-loader.php on line 2850