In that case, you can block your current SIM card and request a new one. To do so, go to this page.
If you request a new SIM card, it will be sent to you by regular mail free of charge. You will of course keep your phone number and your remaining calling credit. To block your device we need some extra information: what was the type of device; where did you lose your device (city); what was the time when you lost it (day and time) and what is the device's IMEI number? The IMEI number is stated on the packaging or purchase invoice of your device.