It looks like a Chamacereus silvestrii, even if it should have more proliferated after 5 years. Chamacereus tends to harden at the base over the years (I believe, due to said proliferation, the bottom part of the plant is no longer required to do photo synthesis after a couple of years, because, by then, light won't reach it anymore.)
It looks like you just watered it before taking the photo. If the base ist still solid then there should be no problem. Give it a nice sunny place at a window (o even better: outside) and a cool, dry and sunny place during winter (Chamacereus is in my experience one of the frost hardiest cactuses, as long it is kept strictly dry from, say, october to april, depending on where you live) and it should proliferate and flourish again.
It's hard to say for sure from the picture, but it doesn't look good. The top part looks pretty good relative to the bottom.
Is it soft or just discolored?
If it's soft I recommend cutting it off above the bad part and let it rest in a shady but airy place for the wound to heal over or "cork".
Find some actual cactus/succulent soil if possible. The current soil looks way too rich and retains water for too long. If you can't find cactus succulent soil, get some regular potting soil and add about 40% perlite or equivalent. Fill the pot to within an inch of the rim.
Bury the healed end of the cutting about an inch in the soil, you may need to prop it up. Water sparingly, and this will depend on the amount of sun, the temperature and humidity.
I'm not sure what species this is, but I suspect it's one of the epiphytic/lithophytic species and is probably less sensitive to overwatering than a true desert cactus.
Going forward, water it when the top few inches of soil are dry, fertilize with half-strength balanced fertilizer a few of times per year during the summer/growth.
Best of luck, it looks like an interesting plant.