Answer: Sounds like a root rot problem, or perhaps a poor environment. Cactus likes bright light, heat and drought. If your cactus is drooping it may have been overwatered, or it may not be getting the amount of sunlight it requires. You can help things along, especially in the winter months, by supplementing the natural light with artificial light in the form of fluorescent light or a gro-light. Try to keep the plant as warm as possible; keep it out of cold drafts, and water thoroughly, letting the soil dry out between waterings. Once or twice a year try soaking the entire pot in a pan or bowl of tepid water until no bubbles emerge, then letting it drain. Don't water again until the soil is completely dry. Most cactus rest during the winter months, won't require much water, and will survive cooler temperatures (55F -65F). They'll need much more warmth and light in the spring, summer, and autumn months. You may want to unpot the plant to check the health of the root system. Sick roots will be an orangish-brown or black; healthy roots should be a creamy white. If you discover root rot, try cutting away the affected roots and repotting in a sharp sand type potting soil.
Hope the above helps revive your cactus!
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