There is no good reason to open your hive during periods of extreme cold. None. Even if you suspect they have died, for whatever reason, opening the hive can typically wait.
Honey bees will form a protective spherical cluster and shiver, the outermost bees rotating with the inner bees, to warm themselves. The bees shiver to generate heat and the outer bees form an insulative layer. Being cold blooded, all bees exhibit some degree of slowing down when they are unable to warm themselves sufficiently. As the temperature continues to drop a protective physiological mechanism called torpor begins: a semi-hibernation, reaction to the cold, that reduces their metabolism and allows them to conserve energy and survive by significantly slowing down their physiological processes. Dependent on cluster size this may affect a larger percentage of the bees surrounding the core of the cluster. The role of the beekeeper is to allow the colony to conserve what heat is available by not opening the hive. In a case where it is known the colony is running out of food wait until the hive and bees have warmed enough to move about and only then briefly open the hive enough to insert frames of honey or other food. During extended periods of cold weather it may take a few days for the bees to warm up and begin to move about freely.
Honey bees’ ability to survive winter temperatures is related to both the formation of a winter cluster as well as physiological mechanisms. Another physiological mechanism involves a sugar within honey bee hemolymph called trehalose. Monosaccharides in honey bee diets are fructose and glucose but the most common circulating sugar in hemolymph is trehalose which is a disaccharide consisting of two glucose molecules. Trehalose acts as a protection to cells by preventing the crystallization of cell membranes and enables the bees to come very close to freezing yet recover without ill effects after warming.