Clumps of hard brown sugar are almost impossible to work with. Once it goes hard, though, there are a few simple ways to return it to its original state. For instance, try placing a piece of bread on top of your brown sugar in an airtight bag. As the bread goes stale, its moisture will transfer to the brown sugar, making the sugar soft again. Apples work in the same way, imparting moisture into the brown sugar in just a day or two. The last method that you can try is microwaving your brown sugar for 25 to 30 seconds — it’s a quick and easy method if you need the brown sugar pronto.
If you want to prevent rock-solid brown sugar in the first place, there is one easy thing that you can do. Put a lemon or orange peel inside an airtight Ziploc bag with the sugar. Make sure the peel has a few traces of pulp. Your brown sugar will stay moist and ready to bake.