My best guess from the photo and your description is too much water. During the winter months cacti go dormant and often require no water at all when grown as a houseplant. I suggest a bright window and no water for 30 days. Sometimes when a plant wilts it is due to root rot from too much water. If the plant does not improve take it out of the of the pot and repot into cacti soil. Water just enough to dampen soil after you transplant. Good luck and Keep Growing, Marianne Binetti