Why ain't my key working? is there anything i can do, how much will it cost?

Hi, my key got damaged yesterday and the plastic keeps coming of the car key everytime i try putting it back together. The key locks, and unlocks the car that's it. When i put it into the igniton, it turns over. and writting comes up saying "Faulty Anti-Theft" I think it's th chip but not to sure? could you help? thanks jessica.