I would remove the sim, and clean it with a dry microfiber cloth and re-insert it. Also try restarting the phone after reinserting it. Just rub the metal contacts on the SIM card until the dull metal looks shiny again. It may just not be making good electrical contact. Don't put any water or liquids on it, just use a dry microfiber cloth to clean it. If this doesn't work, the SIM card could be damaged, or the phone could have hardware damage. Do you have another SIM card to test with your phone. If the SIM card is damaged, you can get a new one replaced from your cellular provider.