Kindly do the following.
1) Create Full backup of your site using a plugin.
2) Re-install your theme and Cornerstone.
Deactivate and Delete xtheme and Cornerstone then install it back again.
3) Re- validate your theme
4) Add the code below in your
wp-config.php file located at the root directory of your site.
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
If it still doesn't work, Kindly provide us your wordpress admin login in