I cna't tell if yo mean the crack is in the bottom edge of the top shell, or if it's on the platron, the bottom shell.
If it's in the top shell, you will have to decide how serious it is. The concerns woudl be whether it's growing, and if it exposes anything internal to the animal. In either of those cases, you will need to find a vet even if you have to travel to do so.
If the crack is in the plastron, I would seek medical help right away.