Depending on your age and your bite, a fracture of this type can occur due to excessive wear to the top (incisal) edge, exposing the softer inner dentin layer of the tooth
. If it is a small fracture
it may be repairable by bonding. A larger fracture
may require a bonded porcelain repair. I agree with Dr. Erfani above that the cause should be carefully evaluated since you may be at risk of the same thing with other teeth. Bruxism, or nightime grinding, may be the problem.