All honey hardens and crystallizes over time, but it need not be discarded. To bring honey back to its translucent, liquid state, Janet Reynolds of Dedham, Mass., uses a pot of simmering water, although you could also use a microwave. Once cooled, use the honey or screw the lid back on for storage. The honey will eventually recrystallize, but it should flow freely for several weeks.
Place the opened jar of honey in a saucepan filled with about an inch of water and place over very low heat, stirring the honey often, until the crystals melt. Alternatively, heat the opened jar in the microwave on high power in 10-second increments, stirring intermittently, until it has liquefied.
Source: Cook's Illustrated - Published January 1, 2004.