Well, first you need to determine if the SIM is actually good. The simplest way would be to try it in another device. If it works in that device then the SIM is good. If it doesn't work in the other device either, then there is probably a problem with the SIM. Should that be the case, and the SIM is bad, you would need to contact your service provider/carrier to get them to send you a new SIM so you can use their service.
However, if the SIM does work in another device, then the problem is with the one that isn't working. Check the SIM port and make sure there is no dust or debris in it. Also check for any bent or broke sections in the port.