When I loaded your home page in Cornerstone, at first, I got a blank content or the content didn't load but there's no error message. I just reloaded it again and the Cornerstone worked fine. I've removed the period after the headline to show you that editing and saving works (see Secure Note).
This usually happens when server resources hit the limits. Please contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Please also ask them if your server resources have been limited this past week or month.