Brown sugar has a tendency to set like a rock when it’s been left to its own devices at the back of a cupboard for a while. While a hammer might seem like the right tool for the job, it could be a little excessive and will probably end up with the kitchen showered in a sugary mess.
A little planning and your cake-baking will be a lot easier second time around. A slice of apple in the sugar packet, left for a couple of days, will soften it completely. A slice of bread, bizarrely, will work just as well.