Crocodoc, which tool that generates the document preview in the Speedgrader, was unavailable this morning, 9/11/15 from about 9:00 AM to 9:30 AM. If you went into the Speedgrader during this time, you would have seen an error message and been unable to preview or mark-up submissions. However, you would have still been able to download the files.
The issue has been resolved as of 9:27 AM. Please see http://status.instructure.com/ for more information.