Hi David, I had exactly the same problem today and came across your post trying to find a solution. It took a bit  searching and trying out a few different things, but in the end I got it working again. Thought I should post here in case you were still experiencing the problem or if someone else finds their Rich Text Editor mysteriously stops working.

And the highly technical solution is... *drumroll* Clear your browser cache.

For your reference I also cleared the DNN cache in the host settings, and restarted the application. Neither seemed to work at the time but perhaps that combo + clearing browser cache is actually what did it. The RTE came back immediately after clearing my Firefox cache.

Hope that helps! :) Cheers, Danielle