There is no draft that I can see in the autosave table. My recommendation which I have been doing with long posts is to store them in a Notepad document in case something like this happens. I'm surprised that the Undo function was unable to restore your text. I don't know how long your post was, but all browsers worth mentioning have limits on post data and data can be lost if the object exceeds a certain size. In Firefox the limit is 4096 characters. Other browsers have higher limits.
From the sound of it, this is either a bug in Ckeditor, Project Tools, or an issue with browser limits and there's not much we can do about it from our end. Still, I will see if I can duplicate the effect.
-- EDIT --
I was unable to duplicate the effect under Firefox, which has the lowest memory of the popular browsers. Using WYSIWYG and an editor that contained more than 4096 characters, I was still able to use the editor's IMG button to insert image URLs into the text. It's possible you accidentally highlighted the text and pasted the image over it. My mouse's scroll wheel occasionally gets wonky and does things like this.