The stem and the root system of this plant is dead. You can try re-rooting the rest of the plant by cutting off the stem above the dead tissue. You need to cut to healthy tissue.

Then let the cut dry out for several days in cool, bright shade.

Then empty out that little pot and fill it with fresh new cactus/succulent soil, preferably with added pumice or perlite. Do not water it. Put the cutting into the soil and the little pot into bright shade. Mist the foliage with water lightly and frequently and wait. The plant can grow new roots. When it has some roots you can start watering the soil. Let the soil just dry out before watering again.

Quite likely overwatering. Try cutting off the green part of the stem and put it in fresh cactus soil, 1:1 sand/sphagnum moss, or sand alone. Let the leaves touch the soil as they root by themselves as well. Keep it slightly damp only.

The stem in the red oval is all dead. You might be able to save and reroot the end if you cut at approximately the orange line.

I'm guessing that is rotting from the bottom up because of over watering but I'm not sure.

The only thing you can do at this point is clip the stem off where it’s still green, let this piece dry out for a day or so and place in a tiny pot with proper potting mix. Do not water until it gets roots. It looks like it was far too wet. If you want to know what potting mix to use, ask here. It would be helpful to you if you read some of the other threads here to find out proper care of cactus and succulents.

The stem and roots are dead, looks like rot from over watering as you're seeing in your cacti. You can cut the stem above the darkened part making sure the cut end is free of any rot, let the end callus in air for several days, and then stick the cutting in a gritty, freely draining mix. It should re-root. The media it's planted in now is too water retentive.

It definitely looks like the stem is rotting to me. I would snip it above the black where the stem is still kind of green, let it callous for a day or two and replant it in a good soil mix.

"The black color has been spreading throughout the stem"... that is rot spreading thru the plant. To save it, you need to cut healthy part ASAP. Sooner = better.

Cut where it is green, and if you see any dark marks in the flesh, keep cutting thin slices off. That is rot and it needs to be eliminated. You may end up with very short stem, but if there is any dark left it will likely continue.

Just in case there isn't any healthy stem left (it should be), leaves will root easily.

Just checking - I posted a comment 1hr ago, but post still didn't move up or showed there is response, yet comment is there.

Something is 'weird' - when I wrote my 2 posts (at 3:19 and 4:29pm yesterday), there were no posts at all. Now I see there are 5 posted before mine, and posted before noon and few even days before - np, just wondering how it happened...