It either needs water or it's dead. Cactus should be watered when the soil is almost dry, not on an arbitrary schedule.
If it looked fine three days ago and now looks like the picture, it's probably dead. Hard to say what may have killed it. There are no obvious pests. You could dig it up to see if there are still roots.
Have there been any recent changes in environment? Like changing windows, inside/outside, etc. When was it watered last? Has it ever been fertilized?
If it was my plant I'd water it heavily for two days in a row and see what happens. Hopefully some water will perk it up.