Hi Elizabeth,
Thanks for writing in! First of all, please make sure that you’re fully updated including X theme and Cornerstone plugin. You can refer to the latest version numbers from here (https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195) and update your X theme and plugins accordingly (https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62).
Also it could be that your under construction may have been corrupted as well. In that case, manually delete the plugin by connecting through your FTP and re-install the under construction plugin. You can locate the plugin folder under the following location (/wp-content/plugins/tco-under-construction).
If you’re still having issues, then check the PHP version running on your server and make sure that you’re using PHP v5.6.x or later to avoid incompatibility issues. Check your server error log to see if you have any errors recorded. You may have to contact your hosting provider if required.
Further you can isolate the issue by disabling your other 3rd party plugins as well. If the issue is resolved, then you can activate your plugins one by one to see which plugin causing the error.
In case if you cannot isolate the issue, provide us with your WordPress login credentials along with your FTP credentials in a secure note to investigate this issue further.
Thanks!