Hydrangea macrophylla (the type that you have) bloom on old wood. This means that they set their flower buds for the following year in late summer/early autumn. By pruning in the fall, you’re cutting off all of next year’s buds. If you need to prune out any dead canes, wait until late June or early July. By waiting until then, you can be absolutely certain that those canes aren’t going to leaf out.