please help with my sick cactus?

I've had this cactus for a few years and it was growing well.
suddenly, it started to dry out and go black at the tips. I stopped watering it and have cut off the dried tips a few times, but the black keeps coming?
I've lost about 20% of the plant.
Is it too late? Or is there some way to save it?

Thanks

 

Does your plant ever get really cold?

 

yes, It was by the window this winter.
I guess I should have moved it.
Is there anything that can be done.
Each month it gets a little shorter?

 

You can only cot it back to where there is NO sign (zero) rot left and hope it recovers. LOTS of light, a little food and some luck are all you need.

 

thanks
I'll keep cutting and cross my fingers.