The other day, I was making cookies to take to a lady I visit teach. My brown sugar canister was empty, so I went digging in the pantry for food storage brown sugar. I found one box and it was ROCK HARD! So I had Zac do some Google-ing and here are the solutions:
IMMEDIATE: Put what you need in a bowl and cover with a damp paper towel. Microwave in brief intervals till softened. Try not to melt it, although mine was a little melty and still worked.
LONG TERM: Put the brown sugar brick in your canister with a piece of bread. Give it a couple of days and you will have normal brown sugar! I shook the canister around each day just to break it up. Take the bread out after a few days - it'll be a brick now, but your brown sugar will be nice and fluffy!!