In most cases this isn't something to be alarmed about. In fact, it means your bees are healthy and they are doing what they should be doing, cleaning out the hive. Bees die all winter long, just not as often as they do in the warmer months. Those dead bees on the ground in front of the hive (easy to see on snow) are just the dead bodies that the rest of the hive is removing in order to keep their home clean. You will notice it even more when we get those odd warm days of 40 or 50 degrees in the middle of winter.
If the bees are still balled up inside the hive or you give the hive a knock and hear a buzz, you should be in the clear. As long as they have food and are protected from the elements, they should be doing just fine, despite the scene out in front of the hive.