Normally I store granulated sugar in a large tote in my pantry. That tote got overfilled with vacuumed sealed packs of tea. I put a few bags of sugar on top of the tote, and they promptly became hard as cement blocks.
I searched online for ways to soften it. One suggestion was to treat it like brown sugar and use a slice of bread. I put it in a container with three slices of bread and sealed it up with no luck.
Another suggestion was to microwave the sugar. I set the timer for 30 seconds and was really surprised at how well it worked. I was able to get half of the block of sugar shaved down with a large spoon. Another 30 seconds and the entire four pound (remember when sugar was in 5 pound bags?) was soft enough to put into the canister.
I have another bag that is just as hard and am glad that I have an easy way to soften it. Throwing it away is not an option, but I like to having a less stressful way to make it useful again.