Hi Waseem,
Thanks for writing in! Are you using any third party Cornerstone plugins? If so, you need to disable them first and test. Also try disabling other 3rd party plugins and test.
If you have a child theme, try switching to your parent X theme to see if your customizations in the child theme related to this issue.
Also check your PHP version running on your server, and make sure to use PHP v5.6.x or later to avoid incompatibility issue.
Further if you’re getting any errors, you can re-install X theme to resolve the issue. Basically you can delete X theme and Cornerstone plugin manually by connecting through your FTP. Then download the latest version of X theme from your Envato account and re-install X theme. All the current settings will be saved in the database, you can safely delete X theme and Cornrerstone plugin.
You can also refer to our documentation here (https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62).
Let us know how it goes.
Thanks!