I had a hydrangea almost exactly like that one last year! It looked very bonny until the flower heads started dying and going a horrible brown colour. The whole thing started to look very sad and slightly fetid. Then when I deadheaded the flowers that were minging no more came. It didn't like winter much and died right back only to return a month ago and start bristling with new shoots. I am an unforgiving sort and fig it up and three it on the compost. I now have a lovely Hebe sitting there..

So, I hoped your hydrangea has better luck than mine!