Some hydrangeas bloom on older wood that has been part of the plant since the previous summer. New wood is the wood that has developed on the plant in the current growing season. This type of hydrangeas will produce flower buds on hydrangea stems around August, September or October for the following summer's blooms. If these stems are disturbed or pruned in the fall, winter or spring, the buds will be pruned and will lead to little or no bloom the following summer.
As noted previously, remove all dead stems from your hydrangeas every year. If the plant is 5 years old or older you can remove up to one third of the older stems. Do this in the summer. If you wish to prune the plant to reduce the size, this should be done in June or July as not to harm the plants bloom for the following year. The problem with this is that the plant will generally return to the previous size. It is best if possible to plant your hydrangea plants in an area where they will not need to be pruned due to their size.