You should contact us as soon as possible via email, by using our online chat facility (You will find the live chat button on our contact page) or by calling our customer support team on 03333 442278, between 9am and 5.30pm, Monday to Friday. If you call from your mobile, you will be charged 1p a minute.
Alternatively, you can bar your SIM online. If you are already signed in to your account, you can simply visit the Bar/unbar your SIM page. If not, visit the Log in page to log in to your account and then visit your 'Network settings' page. Click on the blue 'Bar SIM' button and you will then be able to activate either the 'Lost' or 'Stolen' bar.