Brown sugar goes hard because the moisture in the molasses evaporates, causing the crystals to stick together. You just need to reintroduce that moisture.
The Microwave Quick-Fix
Place the sugar in a microwave-safe bowl and cover with a damp paper towel. Microwave in 20-second bursts, poking with a fork between each, until soft.
The Bread Method (Overnight)
Place a slice of plain white bread in the airtight container with the hard sugar. By morning, the sugar will have absorbed the moisture from the bread, leaving the bread hard and the sugar soft.