Can the hardening of brown sugar be prevented or reversed?

First, hardening can be prevented or at least postponed if the brown sugar  is stored in a way that prevents moisture loss.  Close it tightly in a plastic bag or in an airtight container. 
 
If you google “brown sugar: how to soften,” you’ll find a surprisingly long list of sites that recommend methods for dealing with hardened brown sugar. There are techniques for softening it immediately and for doing so overnight.  Here is one of each:
 
Overnight:  Put the brown sugar you need to soften in a container, add a slice of bread or a damp piece of paper toweling, cover or wrap tightly, and, by morning, your brown sugar “rock” will be soft enough to smash  with a fork. 
 
Instantly:  Heat the amount needed in a 250° oven for a few minutes or in a microwave oven on low with a damp towel over it and a plate or plastic wrap over that for about 1 minute.  Sugar softened in an oven should be used immediately. 
 
Source(s):
How to soften a rock-hard brown sugar? « Do it the Moroccan way “
The Sugar Association
Domino Dark Brown Sugar Packaging and customer service line
(Phone:  1-800-729-4840)