I had just also this experience. I remembered the last steps i made: i configured some things at the ckeditor.
I undoed all the steps and ckeditor was visible again! So i configured step for step again and found the change that made ckeditor unvisible:
It was the addition of the small tag in the format-dropdown. The original list was:
p;h2;h3;h4;h5;h6;pre;address;
and i made it to:
p;h2;h3;h4;h5;h6;pre;address;small;
and then ckeditor is invisible. Strange. But i'm happy i can edit again…