If you store your brown sugar like I do (in a kitchen cabinet with a twist tie), there’s a good chance that it’s dried out on you at least once.
This happens because once you open a new bag, the moisture from the molasses in the sugar begins to evaporate. You can slow the process by storing the open bag in a resealable plastic bag or in the fridge, but this doesn’t prevent hardening entirely, and it doesn’t help you once the brown sugar has already dried out. Fortunately, softening brown sugar is not particularly hard. Here is the method which has given me the most success.
First, break up your brick o’ sugar into smaller clumps as best you can. Drop it, hit it with something (a rolling pin is my preferred bludgeoning tool, but a meat tenderizer would be excellent too), break it up however you choose. When you have some pieces of a more manageable size, estimate how much you will need for your recipe. Here, I’ve overfilled a 1-cup measuring cup with clumps because I need 3/4 cup for my Lemon Cranberry Oatmeal Cookies.
Place your sugar clumps into a glass or ceramic bowl. Wet a paper towel and wring it out so that it’s no longer dripping. Now place the paper towel loosely around the sugar clumps in the bowl.
Microwave your sugar clumps for 1 minute. Take the bowl out of the microwave and carefully uncover the clumps (they may be hot). Using a fork or your fingers, try to squish one of the clumps. If it is still too hard, give it another 30 seconds in the microwave while covered with the paper towel and try again. Repeat until your clumps are soft enough to crumble.
Now you should be able to get back to baking.
*Note that this is only a quick fix. When you soften brown sugar this way, it will probably re-harden in a few hours. Your best bet in the long run is probably to store your brown sugar in an airtight container (or use it up so fast that storage isn’t a problem).