I left for the weekend and returned to find my cactus in dire shape. Living conditions have not changed at all and the deterioration has unfolded over the course of three days.

One of the stalks has blacked, shriveled, and wilted. The other two are fine. The cactus has been in the same place for over a year, I do not believe it is sun damage. The window is south west facing and I have an in-window air conditioner on the other side of that wall, therefore not directly blowing on it. The room temperature is kept between 68-78 degrees throughout the day. The cactus was infected with a mealybug, but I used some spray to rid it of them a while ago.

I think I need to cut the stalk, I don't want the rest of the plant to become infected if it is a disease.