Hi Lisa,
I got error when I check your site:
Above is the current content of your child theme style.css when I check. Due to errors, the reference on the main X theme is wrong resulting to this issue. I have remove the error and you can see the content of your CSS now is this:
/*
Theme Name: X – Child Theme
Theme URI: http://theme.co/x/
Author: Themeco
Author URI: http://theme.co/
Description: Make all of your modifications to X in this child theme.
Version: 1.0.0
Template: x
*/
It is now fixed: https://screencast-o-matic.com/watch/cb6TicI9Bj
Hope this helps.