My casting cracked. How can I fix it?
That depends entirely on the piece and the crack. Some cracks can be fixed relatively easily, while for others it might not be worth it.
If it's a large crack or one that's is in the interior of the piece, you'll probably need to recast. There's nothing you can do to get rid of interior cracks once the piece is cast, but you can try to determine what caused the cracking and improve your process for the next project.
Was the piece cooled too rapidly and thermal shocked? Did it stick to the mold material? These are two common causes of cracking.
If the crack in your piece is not on the interior and if gluing seems appropriate and possible, you might try using a two-part epoxy adhesive.