How do you save your world in MineCraft when reformatting your computer?

On minecraft, I don't have an actual world; something went wrong and my world got deleted. The name was "Banana" and the seed was "glacier." I put that into the "Create New World," and it got me back to my old world (Banana.) Now I have to do that every time I want to access that world. So question number 1, how do you get the world back, and 2, how do you save it while reformatting?