Crystallization refers to the change in state that occurs when an epoxy resin or hardener goes from its liquid form to a solid form. Crystals form for a couple of reasons, including contamination or exposure to cold temperatures.
Yes, you can still use an epoxy resin or hardener that has crystallized, but before doing so, it's important to turn it back into a liquid. This is easy to do simply by setting the closed container of crystallized resin or hardener into a 140-150°F bath of warm water. Change the water, as needed, to maintain this temperature. Stir gently while heating to help break up any remaining crystals.
How to prevent crystallization
Cold and contamination cause crystallization. To keep crystals from forming, seal lids tightly to keep out contaminants, including dust. In addition, store your epoxy resin and hardener in a dry place where the temperature range is 60-90°F. In cold temperatures, be sure to store epoxy and hardener on a raised surface off of the floor, and make sure they're not near outside walls or doors.