It is possible to soften white sugar that has turned hard by breaking it up into smaller pieces or blending it until smooth with a food processor or blender. Sugar hardens when it exposed to even small amounts of moisture.
The Sugar Association recommends breaking up hard sugar pieces with a heavy spoon or meat tenderizer before blending. Because sugar hardens when it is exposed to humidity, keep white sugar from getting hard by storing it in a sealed container. Other methods that work include rubbing the pieces over a fine cheese grater or using a coffee grinder, especially if only a small amount is needed. One sugar manufacturer recommends placing the hardened sugar in a casserole dish and cooking for 15 minutes in an oven pre-heated to 150-200 degrees. After 15 minutes, tap it with a spoon. If the sugar starts to fall off the chunks, place it back in the oven with the heat turned off for an hour. In humid climates, consider placing a piece of bread in the same container as white sugar. The bread absorbs the moisture, keeping the sugar from hardening. Sugar that is properly stored in an air-tight container has an indefinite shelf life.