If the Cactus is completely dead then it's unlikely that you'll be able to revive the cactus and the only thing you can do is get rid of the Cactus.
You can try cutting off damaged or bad roots if the cactus roots have root rot and then replant the cactus and it may come back to life after a few months of continuous care and watering.
To have a better chance at reviving a cactus start replanting it before it gets to the point of being completely dead so if you notice the cactus getting soft and mushy the cactus is likely dying because of root rot and you should cut off the rotted roots and then try replanting it in some fresh soil and watering it again.