You may be a victim of identity theft. This is a type of fraud which takes place when someone you don't know gets hold of your personal details and uses them to access your account.
If you have been a victim of identity theft and your card is still in your possession, you will not have to pay for anything bought on it without your permission. If your card has been stolen, the most you will have to pay is £50, unless it can be shown that you have acted fraudulently or without reasonable care, for example by keeping your PIN number written down with your card.
You must act quickly: contact your credit card company straight away, keep a record of all conversations you have with them, and copies of any letters sent or received; and report the matter to the police, and get a crime reference number.