No, it’s not your fault the economy crashed. Or that consumer preferences changed. Or that new technologies have blown apart your business model. But if you’re the C.E.O., it is your problem. So what are you going to do about it? First-hand stories of disaster (and triumph) from Mark Zuckerberg, Steve Ballmer, Satya Nadella, Jack Welch, Ellen Pao, Richard Branson, and more. (Part 4 of a special series, “The Secret Life of C.E.O.’s.”)