I agree with madpenguin, although you might be able to cut the main stem above the rot and replant the top when it has calloused. Do not water it at all until spring, as with most succulents.
Hi, I have the exact same problem now -- probably shouldn't have watered my cactus in NYC in the winter at all :,) Were you able to cut the top off and revive the plant?
Posts
Btw, it is not a cactus, it is a Euphorbia.