Compose Message Dialog is Blank

I’ve suddenly lost the ability to compose messages. The “compose” box does not appear. Can anyone help? I’ve never run into this before.
Updated CKEditor plugin to latest version. No change.

It appears to be CKEditor or CKEditor plugin that is no longer working for me. When I switched to the old fckphplist editor plugin I was able to get my compose window back again. But I really want to use CKEditor.

@dschaller Use the developer tools option in your browser to see whether there are any http request errors or javascript

In Firefox and Chrome that is Ctrl+Shift+I together. Then navigate to the Content page again.