I spent hours composing my post. Just before I published, I hit Ctrl-Z - and at that very same moment, AutoSave kicked in, and saved my empty post! Please get my post restored!!This is yet one more request that can't be easily fulfilled.
If you, personally, are familiar with Ctrl-Z (Undo), you may also know about Ctrl-Y (Redo).
If you act immediately, you may be able to recover your work.
If you just un did everything in your post - and your work was unsaved - you may be able to re do everything. If, and only if, you act immediately - hit Ctrl-Y, then Save, to recover from your mistake.
If you just learned about your mistake, and you are now reading this post, the latter advice may not help you - this time. Just be prepared, if this happens to you, again.
Post Editor is not part of a Transaction Processing System.
Just as Blogger is not a Content Management System, the Blogger Post Editor is not based on a Transaction Processing System. It is simply not possible to restore blog posts to any given point in time, to reverse a mistake that you may have just made.
If you spend hours without Publishing your new post, and you lose what you've composed by saving an empty post, that's just like saving after you make any other mistake, large or small. Once you Save - either by hitting the button, or by letting AutoSave save for you, when you are composing a new post - what is Saved is what is now in the Draft copy of your unpublished post.
If this is not acceptable, write as much content as possible, publish, then edit.
If you cannot deal with the limitations of AutoSave and Post Editor, disable AutoSave as you compose a large post. As you compose, hit "Save As Draft" regularly - after you have published an initial stub copy of your new post. You may then decide - not AutoSave - when to save as Draft.