Take it away from your other plants and slice it up. Maybe there is something worth saving, maybe not. Even though this is a plant that it would be better to replace rather than spend heroic efforts trying to revive it, it is good practice to see what you can save.
I agree with the other cacti growers. That specific cactus is a goner. Even if you could save one or two areoles, there may already be the presence of the decay organism in those and they would succumb too. I would go ahead, toss the plant and the soil (growing medium) and desinfect the pot before reusing it.
I know this is going to sound very noobish, but which cactus is the dead one?
"And the sun sank again on the grand Australian bush—the nurse and tutor of eccentric minds, the home of the weird, and of much that is different from things in other lands." -- Henry Lawson, The Bush Undertaker
oh no, are they alright now? cut back on watering immediately and cut off any infected areas. hopefully you can save a few