If you’re worried about terrorism, here’s a bigger threat to lose sleep over: an all-out cyberattack.Suddenly, the electricity goes out at the office. Cellphone networks and the internet have also gone black, along with subways and trains.The roads are jammed because traffic lights aren’t working. Credit cards are now just worthless bits of plastic, and A.T.M.s are nothing but hunks of metal. Gas stations can’t pump gas.Banks have lost records of depositors’ accounts. Dam floodgates mysteriously open. Water and sewage treatment plants stop working.