Carnegie Mellon University
If you suspect or have been notified that your financial account number may have been exposed, do the following:
Contact the financial institution that issued your account as soon as possible and ask them to change your account number.

The sooner the number is changed, the lower the risk of fraudulent activity.

Some financial institutions require timely notification as a condition of waiving your liability for fraudulent charges.
Review your financial account for unauthorized activity such as charges, change of billing address, addition of authorized shipping addresses, new card requests, etc. by checking your account online or by examining your most recent paper account statements.

Ask your financial institution's fraud department to help you audit your account for unauthorized activity that may not yet be visible in your online account detail or paper statements (pending charges or requests.)
If unauthorized activity is found, immediately ask your financial institution's fraud department to reverse it and file a report with your local police.  Retain a copy of the police report to document the situation should you need to contact creditors or credit reporting agencies.

Follow up with a written letter to your financial institution in accordance with the dispute settlement procedures of the Fair Credit Billing Act.  Enclose a copy of the police report and follow the format provided by the FTC's Facts for Consumers - Fair Credit Billing.

Ask your financial institution's fraud department to monitor your account for further strange activity.

If possible, enable email, phone, or text message activity alerts for you financial account and check those contact methods regularly.
For steps you can take to reduce your risk of Identity Theft, see Protecting Yourself from Identity Theft.