I have a column cactus(if that's what you call them) that started to have brown parts. The growth on top completely became brown and shriveled. The original body had a brown line down its side that seemed to just lack the green and reminiscent of a tree with its bark taken off(the picture shows the side of the brown running down). I was out of the country for a long time and when I came back there was a new ~1in growth that looks healthy.
What is causing the problem? Overwatering? Underwatering? I usually leave my cactus out in the sun and also where rain can fall onto, so I doubt overwatering was the problem, unless I watered it too much in one instance or something. I live in Minnesota where in the winter the temperature goes below 0 so I have them indoors in comfortable-for-humans temp, but I may have underwatered it during this season(this is what I think is most probable). Also, my cactus had a mealybug problem which I diagnosed, but after my hiatus from my home the problem MAY have reappeared but I am not sure.
And most importantly, how do I help my cactus?
I feel ashamed for neglecting it so much, and although there's no excuses, I want to help it, so thank you if you read all of this.