The first step is to inform your card issuer that your credit card is missing or has been stolen. Your card issuer will block any transactions made to your credit card from the time of your report to prevent any charges to your card. Your card issuer will also proceed to cancel that card. Most credit card issuers have a 24-hour toll-free hotline, so when you notice your card is missing, call them and immediately report it.
Create a contact list with the toll-free hotline numbers of all your card issuers. Store the list in a safe place and also add them as contacts in your mobile phone. This will come in handy if you lose your credit card while you are abroad, as you may need to contact a different number. Having all these numbers with you will make it easier and faster for you to reach your card issuers during times of emergency.