Unprocessed (i.e., raw) honey can become solid and need liquefying before you can use it easily. Rexmo says that all honey will solidify with time; the exception is Tupelo honey. The speed at which honey goes solid depends on what sort of flowers the honey came from. Stirring honey incorporates air, and that will lead it to solidify, too.
To return solid honey to a liquid state, gently heat it in a water bath. Jcannuck says that heating in the microwave will break down the healthful goodies that are found in unprocessed honey.