Far too easily done, which is why you should ALWAYS save as and keep more than one save of each game.
I am currently doing my complete world rebuilds and adding sims as I go, it is a LOT of work and I do not want to risk losing it. I save as A, B, C and D saves and then overwrite A, then overwrite B etc as I am going along so I always have four saves. Yes, I would lose a little if I happened to delete the most recent one but it would not be a major loss compared to everything.
I also occasionally go out of the game and copy my saves somewhere else. Double safety net.