That fatal error points to functions.php file within WordPress core wp-includes folder.
It does not point to any plugin or theme file. Therefore, it’s difficult to know which file is causing error.
You need to do the following.
If you are very sure that you did not edit any theme file. You can access your web server using ftp and start deleting plugins from your server, one at a time, until find the plugin that’s causing the fatal error.
If you have modified the theme and you know which file. You can replace that file with a fresh copy.