If the top is rotted but the roots are all right, just cut the top bit off with a sharp knife. Allowed the cut to dry in a place with good air circulation - you could give it a dusting of Flowers of Sulphur.

I'll upload a photo of one that I did a few years ago, for you to see it growing back.

I don't know why it would have rotted. It could be over watering, or maybe frost damage.

Its a beautiful specimen - hope you're successful.

Pleased to see Bella on the new avatar.