This article is from the Credit cards and Consumer Credit FAQ, by adams@spss.com (Steve Adams) with numerous contributions by others.
Call the issuer right away. Somewhere in the papers that came with
the card is an 800 number to call. If you can't find those papers,
look on a current bill and call the "inquiries" number there. If
you can't find a current bill, look up the bank's number in the
phone book (or call Information) and they'll direct you how to make
a report.
The important thing is to move fast. Once you have reported the
card lost or stolen, you are not liable for any further charges on
the account.
 
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