Check whether your sim is placed properly in the slot. Try to reinsert the Sim. Still if you face the same problem then your phone might be locked to particular Network. To solve this problem you need to get it unlocked. You can unlock your phone using unlock code. You can get the unlock code for your phone from your Network service provider or any third party online vendors like SuperUnlockCodes.com here you need to specify your country, current Network name, IMEI number of your phone. Then they deliver unlock code to you through mail with unlocking instructions. Using this you can unlock your phone from Network lock easily.