Answer: I can't say for sure what is wrong with your hydrangea. Usually they are very trouble-free plants. It may be that the bark was wounded by a lawn mower, insect or chewing rodents and contracted a secondary disease. I don't think it could be frost crack, which affects trees that undergo swift extreme temperature change in winter, but it may be. Without seeing it, I can't say what the chances are for saving the shrub. An arborist or landscaper should be able to help you judge the state of the shrub. You can always propagate new shrubs from the healthy topgrowth, though.
This summer, take stem tip cuttings of two nodes, and remove all but one full leaf. Place cuttings in a flat of moist sand or vermiculite, and place the whole flat in a clear plastic bag to maintain humidity. Place the flat in a bright place out of direct sun, and mist the cuttings daily, more often if they show signs of wilting. They should form roots within 6 weeks, and you can repot them or transplant them to a nursery bed. Hope this helps!
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