How can I save my cactus?

I have had this cactus for almost a year now and water it on a weekly basis. I have 3 cactuses and the other two are fine and I do the exact same with them. In the last week and a half, the one that is dying has become squidgy, wilted and shrivelled as though it has not been watered. Could this be the result of moving it away from the window? It is still in sunlight but further away. Help would be appreciated