Honey will crystallize over a period of time at room temperature, but it is still safe to use. Simply place the whole container in a warm water bath until it becomes liquid again. Alternately you can remove the amount of honey you would like to use and microwave for a short time. If using this method, be careful not to overheat the honey. Do not place a plastic honey container in the microwave.

Additional Resources:

http://www.scienceline.org/2007/04/09/ask-westly-crystallizedhoney/