Fireplace bricks are known to break easily. If your fireplace isn’t in use, it’s best to cover the opening with a heavy blanket for insulation and safety purposes. A broken brick can be quickly repaired by applying fire cement around one or more of its edges before pressing firmly into place until dry overnight. If that doesn’t work, you can purchase an epoxy fire brick adhesive which is specially designed to bond with fire bricks. Allow the adhesive to dry overnight then carefully repaint with a high-temperature stove paint that’s made for such repairs.