Honey crystallizes (solidifies) when it is stored in temperatures between 77°F and 57°F. This is completely normal and will not change anything about the honey except the consistency and appearance. This is an approximate range since different types of honey will crystallize at different temperatures and will take different amounts of time to crystallize.
Only honey will crystallize like this so if you have crystallized honey you can be sure that you have real honey. Fun fact: honey is the second most counter fitted food in the world!
Interestingly, freezing honey will keep it from crystallizing.
If you do have crystallized honey, you can return it to the original liquid state by placing the jar in boiling water or by cooking the jar in the oven. If you decide to cook the jar, make sure to remove the lid before cooking it and only cook the jar at the lowest setting possible for about 20 minutes.