Is my cactus dieing any help please, oh and sensible answers !?

I have a cactus, now I am not sure the type but it has 5 about 3 foot tall towers and each tower is the thickness of an adult fist. Sorry best description. Now 2 of the smaller towers as so to speak went thin near the base of them as if strangling itself to cut them off. well they did cut them selves in half now a 3rd tower is doing it and the main tower has 3 small brown spots on it like a dry patch. I have had this cactus for around 18 years and it would be a shame to lose it. Any ideas please. If you need more info please ask i will try to help