Theoretically, in certain cases it is possible to reinvest the cracked piece and recast it.  But if the mold was damaged while removing the cracked piece, you will need to make a new mold.
And while a damaged piece can be repaired, the amount of time and effort required to do so, combined with the likelihood that the repair may be unsuccessful or the piece may crack again, we suggest a different approach.
Think about what factors may have contributed to the casting in the first place, think about possible ways to avoid those factors in the next casting, and try again.