WYSIWYG editor disappeared in CiviCRM 4.7.16
Well, it works again through magic and voodoo.
All I did was go to the Wordpress Access Control page as Christia recommended and clicked "Save". I didn't modify any settings. Just clicking "Save" seems to have reconnected everything.

http://www.YOUR-ORG.com/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm%2Fadmin%2Faccess%2Fwp-permissions&amp;reset=1