SAT句子提升一道题

SAT句子提升一道题
(Had there been) a power failure,the hospital will run on electricity from its own generators,which can operate for 200 hours.
正确答案是括号里面的改为Should there be,为什么?
咦should是虚拟语气？它啥时候能做虚拟语气？
麻烦再讲一下should这个词吧……我语法实在不好