Seeing some of the comments for revitalizing breads reminds me of revitalizing that pack of brown sugar that has turned into a brick.
I put the bag of hard brown sugar into a plastic ziplock bag. Be sure the bag holding the sugar brick is open. Take a paper towel and fold it into quarters. Dampen the paper towel until it is moist, but not dripping water. Place the moist paper towel in the ziplock bag but outside the brown sugar bag. Zip and let set a couple of days. The brown sugar will soften. Then, periodically, dampen the paper towel and return it to the ziplock bag to keep the brown sugar from turning into a brick again. If it does dry out, repeat the process.
Place a few slices of apple in container with the brown sugar to prevent sugar from hardening.
Our mothers and grandmothers used a piece of bread or apple to try to solve the problem, but there was the possibility of mold imparting an undesired flavor to the sugar, and the solution was short lived.