That sucks. Sorry I don't know the answer. I would have thought it prompted you with an "are you sure" or something like that.
I find your lack of faith... disturbing.
[IMAGE REMOVED FOR PROVOKING NERD RAGE]
Make sure to are using the right place to load the previous game save from like hard drive, memory card, internal 4 gb. If you have cloud save enabled you can recover it from there.
That is a user error like in any game if you wish to play previous level use the time line feature or use a different memory / harddrive .