Contact your bank immediately. You should also report the fraud to the appropriate authority.
Cashier's checks have become an attractive vehicle for fraud when used for payments to consumers. It may take weeks to discover that a cashier's check is fraudulent. In the meantime, the consumer may have irrevocably wired the funds to a scam artist or otherwise used the funds—only to find out later, when the fraud is detected—that the consumer owes the bank the full amount of the cashier's check that had been deposited.
For information on how to protect yourself, read the Office of the Comptroller of the Currency's Consumer Advisory on Avoiding Cashier's Check Fraud.