General food information only, from university Extension sources.
Short answer: Hard brown sugar is not spoiled or bad. It has just dried out. It is only inconvenient.
There are four ways to soften it. They are on this page.
Brown sugar is refined white sugar with molasses mixed back in.
Light and dark brown sugar differ only in how much molasses was added.
Light and dark brown sugar can be used interchangeably. But neither one swaps evenly for white granulated sugar.
Brown sugar holds more moisture than white granulated sugar.
When it is not stored properly, that moisture evaporates.
The moisture that is left works like a glue. It binds the sugar crystals together into hard lumps.
Sugar is hygroscopic. That means it likes to absorb moisture from its surroundings. It will pull moisture out of a moist food stored with it. That is why the fixes below work.
Put a slice of soft bread in the container with the hard sugar.
The sugar pulls the bread's moisture into itself. Then the sugar softens.
You can keep a slice of bread stored with your brown sugar. Replace the slice when it gets hard.
Put the sugar in a plastic bag, a tight-fitting container, or a jar with a tight lid.
Add an apple slice or a piece of bread.
Check it now and then. Make sure the apple or bread has not dried out or turned moldy.
Place a dampened paper towel over a microwave-safe bowl of hard brown sugar. The towel should not be dripping wet.
Microwave for 30 seconds. Then fluff the sugar with a fork.
If it is still hard, use 10-second bursts. Fluff in between each burst. Stop when it is as soft as you want.
Spread the sugar out as best you can on a baking sheet.
Heat it in a slow oven, 250°F to 300°F, until it softens.
Take it out and measure it while it is still warm. It will harden again as it cools.
Store brown sugar in an airtight container.
The container traps the evaporated moisture. It returns that moisture to the sugar.
Penn State Extension, “Storing Staple Ingredients in the Kitchen.”
Kansas State University Research and Extension, “Pantry Ponderings: Brown Sugar,” Johnson County Extension.
General food information only, from university Extension sources.