No SIM

How do I get my phone to work again? I have iPhone X, was working fine this morning, this afternoon it decided that there was no SIM card installed. I went to the ATT store and they updated software and tried a new SIM but was unable to correct the issue. I tried calling apple, but instead of calling the app said they would call me, I’m assuming that my phone didn’t ring because it doesn’t recognize that there is a SIM card in it. Any suggestions?