SIM not recognized

If a restart and reseating don't help: Settings > General > About and wait to see if a carrier settings update popup appears. Then try Settings > General > Transfer or Reset > Reset > Reset Network Settings. Trying the SIM in another phone (or another SIM in this one) tells you whether the SIM or the iPhone is at fault.

Try a force-restart, and try re-seating the SIM.

Then…. Contact your carrier first, and if they can’t resolve this then check with Apple Support.