It was probably caused by dryness or bad storage. The crack goes all the way from the sounding board hole to the bottom edge of the guitar (assuming it's standing).
Take it to a professional. Or leave it alone. I don't think it's a good idea to tackle it on your own, both due to the difficulty of the repair and the possibility that there is other damage that you may not know about.
That being said, here are some detailed examples of what a repair might entail