i have had this cactus for about 20 years. it is about 3 inches in diameter and about 5 inches tall with one small arm coming out of the base. it has flowered once ( many years ago) with small flame red blooms. it's been very healthy growing about a half centimeter per year 'till just recently. the lower half is grey-brown and getting soft. can it be saved? what's wrong with it?
Sounds like overwatering... Make sure it's not sitting in any water and stop watering it. Also place it on a tissue and this may help soak up excess water.
It could also be a fungal problem, i'm not sure how you would sort this out. I'm no expert!
I notice that you posted this some time ago, so my reply is probably too late for your cacti, but if you're about let me know how you (and your cacti) got on! :)