I am using Drupal 7.56 and have just upgraded from CiviCRM 4.7.20 to 4.7.21 with a number of significant issues that will require that I revert to 4.7.20. When I try to display System Status - Warning nothing is displayed. When I try to continue to use a previous email, nothing displays. I don't have the luxury of doing anything more to understand the issues since I now need to revert to 4.7.20.
This is my answer pasted from this question
We had one site that had 'lost' all it angular pages, eg CiviMail and System Status.
In our case the angular cache files were returning 404 on the New Mailing page. (and 'inspector' was showing a lot of red angular errors)
It was building the cache files in sites/default/files/civicrm/images, but then searching for them in sites/default/files/civicrm/persist/contribute
The issue here was a mismatch between the resource url setting for Image Upload URL and the directory setting for Image Directory. Strangely (!!) these folders are being used for caching of angular files.
Try removing content of templates_c folder, location: sites/all/default/files/civicrm/templates_c