Firstly, kindly double check that if you have recently updated X or Cornerstone to the latest version that you have both of the updated as it will cause an incompatibility issue that results to some features not working as they are expected to.
If both are updated and the Global CSS is still not present, try 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.
In case after doing the troubleshooting steps below and you are still not able to see the Global CSS, please provide us with the admin access to your site in a Secure Note: