Is it soft where it is dark and right above it where it is slightly yellowish?

I actually haven't given it a good solid push in to see just how soft it is because it has thorns...lol. What I have noticed is that since I've let it completely dry out, it's lost part of it's root system and half of the plant lifts up when I touch the cactus and it looks like the roots have dried up and broken. I don't know whether that's due to rot, but I'm imagining it is. I will go out tomorrow morning with a pair of gloves and give it a good going over, checking for soft spots.

Thank you for your time with this.

I think that if you find rot that one can cut the bad tissuee off and cut till one has absolutely no discoloration, dust it with cinnamon ( great fungicide) let it callus for a good week or so till it has made a hard skin and then plant it back in a pot of fast draining soil and keep it out of rain and watyering till it roots. might be awhile. A little misting is alright but no watering. The cactus needs roots to take up water.. You could use rocks to help hold it up.

Wait till others chime in. There are others that know more than me.

Thank you guys, so much, for your help. I went out and did some pressure tests on the cactus. I guess I've let it dry out well enough, because I didn't feel any mush, but it looks like about half the roots on one side of the cactus are gone. I can lift up half of the plant, but the other side is still anchored. If it is rot, it doesn't seem to be spreading.

Do you tnink it best that I just let it continue to dry out for 3-4 weeks and then resume monthly watering? Since the rot isn't spreading, do you think it's going to be safe?

If the rot spreads, I'll do the surgery to save the plant though. I'll probably need more advice at the time.

Thanks again!