Hello there! I understand how frustrating it is when brown sugar turns into a solid brick, especially when you are pressed for time with a barbecue rub.
To fix this right away for your cooking today, you can place the hard brown sugar in a microwave-safe bowl alongside a small, damp paper towel. Microwave it in short bursts of twenty seconds, checking frequently, until it softens up completely. To prevent this from happening in the future, always store your brown sugar in an airtight container with a slice of fresh bread or a specialized terra cotta brown sugar saver disk, which will maintain the moisture balance.