Perhaps I missed it, but I don't think Camtasia automatically archives versions of projects.
I know that this doesn't with your current predicament, but I've made it a habit to create a new version of a project (named v1, v2, etc.) every time I do any significant editing. That way I have an archive of my previous work that I can manage as necessary. Since they're all separate files, there's no possibility of them being overwritten by newer work. When a project is complete, I move the old versions to an archive folder to keep the Camtasia projects folder clean and current.