I think the operative word is "crack". That can cover a pretty wide
range of problems. A hairline crack that isn't wide enough to have
let air or debris get in can often be refired and repaired. I have
done it many times myself. I would say the cleaner the piece has been
kept and the sooner the repair is done the better.
A "crack" that is more like a fissure, is made of old enamel that
may have a different firing time than what you are used to, has let
air in and oxidized the metal underneath, has collected dust or
debris in the crack or lost some of the enamel is a different thing.
These are all problems that may or may not be repaired.