Question: Is there a way to salvage brown sugar that has turned to stone?
Yes. You need to get moisture into the brown sugar.
I put the brown sugar into one of those heavy Ziploc bags. Then I take a paper towel and moisten it. I squeeze out all of the excess moisture from the towel and add the towel to the bag with the sugar. You can also add several apple slices. I seal it and leave it in there. After a day, I check to see if it needs more moisture. If it does, I dampen the towel again or add another apple slice.
For an instant solution, you can microwave sugar on high for a few minutes, but be sure to use it quickly because it hardens up again in almost a flash.
Brown sugar turns to stone if not sealed airtight because the film of moist molasses around the sugar crystals dries up, hardening like cement. In the future, put your brown sugar in a sealed, screw-top jar. That way, you will always have soft, moist brown sugar.