Question
I recently left my cactus in the bathroom for a few weeks. It has now gone soft and moldy and I think it's dying. Please help. It's nearly 3 feet tall and I'm so fond of it. It is soft allover and has big white mold spots on it now. Thanks

Answer
I also think it is dying.  Excessive moisture especially when a cactus is dormant will most definitely kill it, tho I suspect the rot began even before, most likely from over watering, and the bathroom did it in.

If your plant has arms, or if the top isn't soft and neither has rot you could cut them off the main plant with sterilized scissors and let them callous over.  Dispose of the main soft, moldy plant and the soil it's in.  Sterilize the pot.

If you think you can salvage anything, and that means segments with not sign of rot, no softness or mold, with healthy green and white vasculature, let me know. If not, you do need to dispose of all of it.  If you have  picture please send it.  It doesn't sound like anything is viable, but you can try to give it a go.  I can help you with that if there is any healthy plant left.  If not, at least you've learned about excessive moisture and can apply it to another cactus that you will become fond of as well.