I'm afraid to say your cactus is dying. The roots may have died from being in the heavy, wet soil, and now the rest of the plant is going. Next time use a much grittier soil mix with half or less bagged cactus mix and the rest perlite or rough gravel. Also, let the soil dry out thoroughly between waterings. Sorry!

Shoot. Alright thank you both! Is there any point in trying to replant or cutting it back? In new soil conditions or from a cutting? Can I save it by leaving it to dry? It still has an extensive root system that seems intact

If the roots are intact and alive you might have a chance. You would need to cut off the discolored top back to healthy green tissue. It would be a good idea to get it out of that soil and into something gritty. That would give you a chance to inspect the roots and see if they are actually healthy. If the roots and base of the stem are dead, you would need to cut them off too, and root the green section. Make sure there is no dead tissue left, and let it sit in a dry spot for about a week to let the ends callous. Then put the bottom end into dry soil and don't water it at first. Start giving it small splashes of water only if it starts growing roots. The chances of success are very slim, but you have nothing to lose at this point, so you might as well try. Good luck!

So I keep the bottom portion of the plant or the top portion? Thanks for answering all these questions by the way. The roots are white and all attached to the plant is that a good sign?

Keep the bottom portion (the green part). It sounds like the roots are okay, so don't cut on the bottom of the plant, just pot it in gritty soil mix. You need to remove the discolored top part so whatever is happening there doesn't spread into the green area.

I actually have no idea if this cactus can be salvaged... To have a chance, you will need at least some healthy tissue... To be honest, that cactus looks a little too far gone. If it is possible at all to revive, it will be very tricky.

I actually would recommend buying a new plant. It will be easier, and you can get a fresh start.

On the other hand, if you really, really want to save it, you will need to start by searching for firm untainted tissue. Just feel along the length of the cactus, and squeeze slightly. From the picture, it looks like there is an area of green left. That would be the ideal area to start checking. If you find a firm spot, cut the top off the cactus at that spot, cutting lower and lower by slices until you reach good solid tissue with as little brown as you can get. Use a very sharp knife. From the picture it looks like the base is also rotted. You will need to cut off all the roots, and cut up toward the top of the cactus slice by slice until you find the same green firm tissue. At this point, you should only have a tiny cylinder of cactus left. Maybe even less than an inch high. You will need to put it somewhere warm and dry (with no soil) for several days. That will let the wounds callous over. After that, just lay the section of cactus on top in a pot of dry dry dry dry dry dry dry 50/50 cactus soil and perlite. DO NOT WATER UNTIL ROOTS FORM. You have absolutely one chance to revive that cactus, and if you water it, it will then rot into nothing. The cutting should take maybe a couple months to root. Do not disturb it while it is rooting. Doing this will cause fine roots to break, and the rooting will take longer. After you are sure there are roots, you can water cautiously.

I don't mean to be pessimistic, but the chances for this working are very slim. Don't blame yourself if it doesn't work, and never give up!

The best of luck to you!

~palmsandsnow