Help, My Peruvian Torch cactus is dying!! : /?

My Peruvian torch cactus is dying from the top down. It started about 2 months ago. The top is shriveled and black and the rest of the cactus has yellow splotching and it gets unhealthier looking and more shriveled as you get toward the top. It's 1.5 ft tall, and the only healthy part is the bottom 3" that is completely full and has minimal yellow splotching. I have a pic of my cactus but can't post it here.