Contact the issuer of the card if you suspect fraud. Credit card issuers typically have a 24-hour hotline for consumers to report fraud and theft. By law, a consumer's maximum liability is $50 per credit card; once they have reported a loss or theft of a credit card to the issuer, consumers have no further responsibility for unauthorized charges. If the fraud involves a person's credit card number, but not the card itself, the consumer has no liability for unauthorized use. If an ATM or debit card is lost or stolen, the amount of money someone could lose depends on how quickly they alert the issuer -- if they wait too long, they could lose everything in the account.
— Courtesy of the Better Business Bureau