The bottom of my cactus rotted away but the top is still angelic. What can I do to stockpile my cactus from dieing? ...
Answers: It sounds like you watered it too much. Is this contained by a pot outside or in? But anyway if it be insect damage the evidence would be more conspicuous then merely the base rotting, so that simply leads me support to over watering.
If the 'good' part have separated from the roots, there's nothing you can do. The plant is motionless. Being a cactus, and not needing profoundly of water, the 'good' division won't show you that it's dead for some time. If the bottom piece rotted, it was given too much sea.