By process of elimination you should be quickly able to narrow down the cause to either a locked/defective phone, or a faulty SIM. If the issue appears to be a faulty SIM visit your neared network provider store to get a replacement SIM Card. If your phone displays this message then try resetting your mobile, turn your mobile off, remove the battery, and ensure your SIM is securely in place. Turn your mobile on again. If you still get this message try your SIM in another working phone - if the SIM works fine in another phone then the issue may be a defective SIM If other SIMs also will not work in your phone then there is a handset fault, or it has been blocked.