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.