Winter kill. I live just north of Concord, NH, and we got down well below zero this past winter with no snowfall, and down below -20 after there was a fair amount of snow on the ground. The previous winter was only a bit less harsh and so many plants were damaged by cold. You are even farther north than I am and so experienced the same type of cold.

This type of Hydrangea, H. macrophylla, has older types that only bloom in spring on old wood, and those wouldn't bloom at all since all of last year's stems were killed off by the extreme cold. There are also some newer types that bloom on old branches from previous years as well as on new growth from stems that are growing for the first time this year, but they will be slow to bloom since they are starting from the ground. Usually the old wood and its buds are only hardy to - 10 to -15, so in milder winters it survives, while in really harsh winters such as the last two, these shrubs have to start with all new wood since the old wood has been killed to the ground.

You can either live with having many blooms only some years, or you can plant either Hydrangea arborescens varieties like Annabelle (white) or Invincibelle Spirit (pink), or Hydrangea paniculata varieties such as Limelight, Little Lime, Quickfire, Little QF, Bobo, Pinky Winky, Vanilla Strawberry, etc. All these start in some shade of pale green to white and slowly change over the course of several months to various shades of pink.