That happens sometimes, ever since version 1.0. It is so random an occurrence I doubt it will ever be fixed.

You also have a previous auto saved game (a backup of the auto-saved game when the program auto-saves), so you can go back to that (if you haven't played so long as to overwrite it with the bugged auto-save.

If not, you are out of luck with this game. There is no way you could ever dig yourself out of that hole, well, bottomless pit.

You should also manually save your game periodically with a unique filename. It might just save you some frustration.