Brown sugar can be a bit problematic because, when left unattended, the moist sugar can quickly transform into a rock-like state. This happens because the sugar dries out and the molasses used to keep it brown (most brown sugars are white sugars, with molasses added back) hardens and sticks it together like glue. There are several ways to avoid this problem. The easiest is by storing the soft sugar in an airtight container or ziplock bag. If it is already looking a little dry, use a terra cotta sugar disk, which will gradually release moisture into the sugar.
If your sugar has already hardened, you can either zap it in the microwave (short term solution) or add some moisture back to it by transferring it to an airtight container or bag with something slightly moist, like a damp paper towel or a sugar disc, and occasionally working with it to break it up.