I don't use it. Put in it dry gritty mix. Put the plant away in a light (not sunny) place and gorget it for a couple of month.(Sometimes a little spray) If healthy it will root again.

Fall is coming, when the rain starts all my cacti are going in a mini-greenhouse for their winter rest. Should I let it go dormant, or should I keep it in the house and keep it growing?

What sort of cactus do you have? Roots don't generally just fall off unless you've got rot issues, which would need to be cut out and allowed to callous. In any case, it'd probably best to let the cactus go dormant for now.

It's Mammillaria prolifera a plant sensitive to rot. I checked out the base and it looked free of rot.

Why can't I see the pic? Am I missing something....

Never attached a pic, didn't think it was necessary.