Thank you for writing in, did this issue happen right just after you installed a new plugin? I am seeing a 500 Internal Server Error when I try to open the Cornerstone. This issue is either a PHP fatal error or your site is reaching the PHP memory limit. More likely the later.
You can increase your PHP memory limit by following the guide provided here. Or you can contact your hosting how to increase the PHP memory limit effectively.
Let us know how it goes,