I updated to Cornerstone 2.1.3 today and it seems to be working ok. Thankfully!
Last week I updated to Cornerstone 2.1.2 and it did not work at all. Only pages with non-Cornerstone content would display. A difference for me was I couldn’t find Cornerstone in my plugins at all.
I followed these steps to start again.
1: Deactivate and delete X
2: Make sure you don’t have a “cornerstone” folder within your wp-content/plugins folder via FTP.
3: Download the latest copy of X via your Apex dashboard
4: Upload the zip via your wp-admin > appearance > themes > add new > upload
5: Upload and activate.
6: The latest Cornerstone will now automatically install and you should be back up and running.
This worked for me with 2.1.2. Maybe you could try that while you’re waiting for help.