Is your brown sugar hardened?
I always place a slice of bread on top and reseal the container. It will be moist the next day .
If you can't wait that long, place the hard sugar in a bowl in the microwave. Next to it, place a bowl of water. Microwave one minute and check the sugar. If it's not quite ready, do it for another 30 seconds and check. If not ready, try another 30 seconds. Careful not to melt your sugar.