I am no expert but I am growing 4 Christmas/Easter cactuses at this time. One I have had for 3 years. When the same thing happened to mine, I took it out of the pot only to see it was not draining as it should and so it rotted at the base. I have since repotted it in better draining soil and it is doing great with new growth and everything. Seems I caught it just at the right time before the rot killed everything. I am happy since it is a treasured gift from a gardening friend. I moved it from the a/c indoors to the greenhouse where it gets mottled sunlight, more humidity, and heat. I miss my plant inside the house and will probably acclimate it come fall and hope for blooms again. But it seems happier outside in the heat and humidity. I do have trouble with yellowing leaves though.
My newest cuttings I got in March and April have tripled in size. They are in half soil and half perlite and sit next to the above established cactus in the heat and humidity. They all get frequent misting but watered lightly only once a week.