I think the proper question to ask is if they are still alive.
You can test for this by taking a pocket knife (is my age showing here?) and cut the bark back to an area of the trunk-about shoulder height. If the layer below the bark which is called the cambium layer is greenish then the tree is still alive and the tree should put out some new growth this summer. Make sure the trees that are alive are watered and feed in the normal manner. Once you start to see a flourish of new growth you can cut the old limbs back to where you start seeing a green cambium layer.