Hardened brown sugar can be softened by adding moisture back into it. One effective method is to place a piece of bread or a slice of apple in the airtight container with the brown sugar for a day or two. The sugar will absorb the moisture from the bread or apple and soften. Another quick method is to microwave the brown sugar in a microwave-safe bowl, covered with a damp paper towel, for 20-30 seconds or until it softens. Be cautious not to overheat as it can melt.