UPDATE: 10/22/16, 12:00 AM
The issue has been resolved and Canvas has updated their Status page: http://status.instructure.com/
ORIGINAL MESSAGE: 10/21/16, 10:45 AM
Document previews in the Speedgrader and Files tools may not load due to issues stemming from DDoS (Distributed Denial of Service) attacks that affected major DNS providers this morning.
Updated information available on the Canvas Status page: http://status.instructure.com/
For more information about this DDoS attack, in general, here is an article from Techcrunch: https://techcrunch.com/2016/10/21/many-sites-including-twitter-and-spotify-suffering-outage/
The bCourses team will update this news article once these issues have been resolved.