I am editing this question to provide more information. I am running CiviCRM 4.7.8 with WordPress 4.5.3. We are using php 5.6.23.
When selecting "New Mailing" from the "Mailings" menu, the screen is blank. Using inspector and console I can see that there are more than 70 errors. The first error is on line 270 of the angular.js file. It seems that there is a comment in the file that is not commented out. All other errors are related to angular, mostly angular is not defined.
So we are unable to do a new mailing. We used this feature early last week and it worked fine. We did not upgrade or install anything new. I tried to turn off extensions, but that had no impact. The extensions were installed prior so no reason to expect that is causing the problem.
Any ideas what would cause this and how to troubleshoot.
Update: We did a fresh install of civicrm, but that did not solve this problem. Also, we have a staging version, which is the same as production without data (a different database) and staging works, just as production worked a few days ago.
When I go into Unscheduled reports, all of my attempts to start a new mailing are listed, but when I click the option to "Continue" I get the same error message.