I have logged in and it seems that the post shown in the screenshot were edited outside of Cornerstone. Did you accidentally switch editors? Please be advise that you are not supposed to switch editor because it might corrupt editors data which might no longer allow you to edit the post in the Pro Editor. To resolve this, you will have to switch back to the Pro Editor tab, ignore the warning and see if you can still load it in the preview. If you are seeing a blank post in the preview, the editors data were corrupted already and you will no longer be able to edit it. You will have to recreate the post again.
Hope this helps.