You can easily fix that and rehydrate the sugar by adding a crust of bread or cracker to the sugar container, covering and letting it sit for an hour or more.
The result will be soft sugar that you can easily scoop for your favorite recipe.
If you don’t use the brown sugar often, consider storing it in a vacuum-sealed container to maintain freshness. That will remove the air from the container as well as keep air out. This is the best storage solution for the long term, but it’s not very convenient if you bake often, so let the default be an airtight canister.