Sorry for all the confusion, we need to see the error first hand so we need to try to login to your wp-admin. You did provided the wp-admin link above, but you did not provide a username and password though. Please provide us login credentials in secure note.
Anyways, most of the time The White Screen issue is cause by a plugin. Please do a testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time. Or if you can't access the Dashboard anymore, you can rename the plugins folder on /wp-content/ directory, this will deactivate all your plugins.
Also please edit your wp-config.php file and change the line:
define( 'WP_DEBUG', false);
define( 'WP_DEBUG', true );
What this does is it will give you the error message instead of just a white dead page. Which can possibly point us to the cause of error.
Another thing, on your .htaccess file the block there seems to duplicated, was that intended? If not, please address that.
Lastly, if you have any caching feature please purge it and deactivate it while we are still dealing with this issue.
Update your XTheme, Cornerstone, and all extensions you have installed. You can refer for the latest versions here. If the issue shows up let us investigate it (do not restore to previous version)
Let us know how it goes,