The Garden Doctor will venture a guess that overwatering was the culprit. Cacti and succulents are hardy plants except when they're exposed to too much water; root rot can quickly do in even the healthiest plants. Check the rest of your plants, and if you find any that are showing signs of rot, cut it off, allow it to dry, and reroot in fresh cactus mix. Replace the soil, perhaps with a soil mix made especially for cacti and succulents. These soils are fast-draining, allowing the plant to take a drink when you water. The excess water is quickly released from the pot. Let the soil dry out completely between waterings. In winter, this means you could be watering every other week or even less often.