No bees are around in the winter due to the pollen disappearing. This is the opposite to warmer months, when there is plenty of pollen available. Beekeepers actually do very little to keep the bees happy during the winter months. Bees have been around a lot longer than beekeepers, and have developed their own systems for keeping warm.
The Occasional Flight - If you were to look at a beehive during the winter months, you might conclude that there are no bees in the hive, because of the complete lack of activity around the hive. If you were to sit there long enough, however, you would eventually see bees emerging from the hive. On a warm day, every month or so, bees will emerge from the hive and take a flight. The main purpose is to empty their bowels, as they prefer to keep the hive clean.
Winter Coping - During the winter months, bees make it through the winter by banding together, literally. They gather in a tight ball around the queen and the honey, and just like holding another person for warmth, the mass of bees crammed together provides warmth. The bees on the outside rotate inward, so that no bee gets too cold and dies as a result.
Honey - Another thing that bees do similar to humans to stay warm is to shiver. The bees on the inside of the cluster shiver their bodies, which generates heat that then radiates outward. The shivering requires a lot of energy, however, and so the bees eat the honey to keep their energy up. This is part of the reason that they cluster around the honey.
Beekeepers - The beekeepers have only a limited role in the entire process. Beekeepers take honey from the bees during the summer, to eat, but they must also leave the bees with enough honey to survive through the winter months. Beekeepers also periodically check the hives and their honey stores, and if they are running low, they may provide the bees with more honey to help them survive. Beekeepers may also need to protect the hives from the elements if the hives are not already adequately protected.