After updating my Civi installation from 4.6.x to 4.6.22, my CKEditor was broken (blank screen in edit window, all areas).
After checking my js console, I found my justify plugin folder was missing:
../civicrm/packages/ckeditor/plugins/justify <= folder missing
I downloaded the plugin from here: http://ckeditor.com/addon/justify
Expand the .zip in plugins folder, it will create the justify folder and fixes the problem.