Answer: Hydrangea macrophylla 'Monink' is considered winter hardy into zone 5, however it sounds like your plant is suffering from winter dieback -- performing as root hardy but not bud hardy. Since it blooms only on old wood, losing those buds/stems from the year before means fewer (or no) blooms.
With hydrangeas, location is very important. It might do better in a more sheltered spot with protection from winter wind. In spring, once it begins to swell, cover it if a frost or freeze threatens.
Also, you might try erecting a windbreak for it each winter, or, you could try wrapping it in late fall to protect it. Some gardeners wrap it in burlap or a cylinder of chicken wire, then stuff some non-packing insulating material such as straw or dry oak leaves inside. Top with a piece of plastic or tarp to keep the rain out, but do not wrap it tight because air needs to circulate. Do this in late fall after it has gone dormant and remove it in early spring before it starts to grow.
Another option to consider would be to plant one of the hydrangeas such as Endless Summer, these bloom on both old and new wood. The old wood blooms at the normal time, followed by blooms on the new wood of the year. This way, even if winter kills it to the ground, you would still get some flowers every summer.
I hope you get flowers next year!
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