Answer: There are several reasons for a cactus to rot, but probably the most common is overwatering which results in root rot. It is particularly easy to overwater them while they are setting flower buds because during this period they do not make much other growth. Unfortunately, I would suspect that is what has happened to your plant. Basic care includes using a very well drained soilless or cactus potting mix, following a "water well and let dry thoroughly then water again" routine, keeping the plant in full bright sun, and allowing it a cool (and drier) rest period of two to three months in winter during which it will set buds. Another possibility could actually be underwatering, and yet another could be insect pests, but I would first suspect the overwatering.
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