I just did a standard plugin update via the dashboard and Cornerstone failed AND removed itself.
“The plugin cornerstone/cornerstone.php has been deactivated due to an error: Plugin file does not exist.”
Looking at files it appears that the update created/renamed the “cornerstone” folder to one called “cornerstonecornerstone”
Looks like this last update is tweaked?
Renaming the folder and re-activating the plugin seems to fix.