Honey will eventually crystalize and granulate over time.  This is a normal process and has a lot to do with the type of nectar the honey bees collected to make the honey.  There is nothing wrong with granulated honey and many people around the world enjoy using granulated honey as a sweetener.  

Honey can be decrystalized by placing the jar in a warm (not boiling) water bath until the honey is liquid again.  You can also use a food dehydrator, using a setting lower than 120* F.  DO NOT microwave honey, because doing so is likely to overheat the honey and destroy the beneficial enzymes found in it.