Q.how to rejuvenate a dying fairy castle cactus
this is apparently a very old cactus; it is about 7-8 ft tall in a 16 inch tall pot. it was standing very upright until it suddenly began “drooping” from the top and looking sick.

Certified GKH Gardening Expert
Check your growing conditions with the tips in this article:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
Is it showing other signs of distress in addition to drooping? This article details how to trim back a cactus, which is what I would recommend: https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-pruning-info.htm