Answer: According to your zip code you are gardening in zone 6A or the coldest part of zone 6. In an exposed or windy spot this could actually be as cold as zone 5, which is just too cold for most hydrangeas to be able to bloom -- although they may be root hardy. So I suspect your plant is suffering excessive winter damage and/or it is being pruned improperly.
The bigleaf hydrangeas (H. macrophylla) bloom on wood that grew the year before. The only "safe" time to prune is right after it blooms in the summer. Pruning at other times of year will remove flowering wood or buds. If it dies back to the ground in the winter, and you have to prune it back as a result, it will not bloom. It your routinely prune it back hard in fall or spring, it will not bloom. And if oscillating spring temperatures result in freeze damage to the buds and new growth, it will not bloom.
You might be able to protect it for the winter by enclosing it in a wire mesh cylinder stuffed with dry straw or autumn oak leaves, top that with a tarp to keep it dry. (Do not enclose it completely because it needs some air circulation to prevent condensation.) Remove this gradually in the spring.
Alternatively, you could try one of the new cultivars such as Endless Summer which can bloom on both old wood and on new growth of the season. Be sure to plant it in a sheltered spot with protection from winter winds. I hope this helps you trouble shoot.
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