How to tell if iPhone's SIM port is working even though it's locked?
This is what I did to solve the problem.

Powered the phone down.
I removed the SIM card.
Use a small paper clip and insert in the sim card hole and push firmly. Don't use a large paper clip. While they fit the hole, are too large to push on the mechansim that pops the card up.
I lightly cleaned the SIM card contacts with an alcohol prep swab (the kind they use when you get a shot). Just so you know, I did try dry cleaning the card with a soft cloth at first, but it didn't work for me. Try it, if you don't have the prep swab, it might work for you.
I then made sure the card was dry of the alcohol before I reinserted the card in the phone.

Restarted the phone.

This solved my problem. The phone booted up normally.
I hope it solves your problem too.