To get things back to normal, sometimes all you have to do is remove and reload the SIM card on your iPhone X. If your iPhone system is unable to read the SIM card due to a minor fault, or if the SIM card is accidentally detached, this might be the solution. Here's how you do it:

1. Shut off your iPhone altogether.
2. If you don't have a little paperclip, use the SIM eject tool to unlock the SIM tray by putting it into the tiny hole or slot supplied.
3. When the SIM card tray pops out, remove it.
4. Remove the SIM card from the SIM tray and inspect it for scratches or other evident indications of damage.
5. Replace the SIM card in the SIM card slot if there isn't one. Make sure the gold contacts on the SIM card are pointing downwards.
6. To secure the tray, press down on it.