If you've lost a credit card or believe it was stolen, you should:
Report your lost credit card to the card issuer as soon as you notice it's missing. This can be done with a simple phone call.
If the credit card issuer is located in Belgium or is a belgian bank, you will also need to notify them with an official letter.
The Card issuer will cancel any transaction issued with your card and you will be covered for any fraudulous transactions that may have been made with your Card.
If you believe your credit card was stolen, you will need to report it to the local police station.
Other useful tips
You should sign your credit card as soon as you receive it to prevent any one else of using it.
Never lend your credit card to anybody
Make sure nobody sees you while you're dialing your PIN code (Personal Identification Code) when using your credit card or getting cash at an ATM.
If your credit card gets stuck in an ATM, do not accept any help from people you don't know: contact the Card issuer
Do not leave any credit card receipts that you don't need where they can be found: if you don't need them, destroy them.
Do not use your credit card for purchasing items on unsecured websites or on website from unknow reputation. The browser your using for online transaction should display a lock to indicate that the connection is secure.
Sign your traveller checks once when you buy them. Countersign the bottom of the checks only when you use them.