Have you paid your bills for the last 4 months? Do you have a SIM card in yout phone? 

Yes and yes.  I reinserted the SIM card but it didn’t help.  I did notice that my SD card isn’t working either.  They both worked fine 4 months ago.

Have you tried your SIM card in another phone to see if the SIM card is the issue?

The SIM card was transferred from my old phone to this one in December.   It worked on the old phone, and it worked fine on the new one also in December.  Then I left the country for 4 months, and somehow it’s no longer working.

Is your old phone still working? If yes, can you put it back in the old phone to see if it works?

Yes, the SIM still works in my old phone.  Just not in the new phone.  Yet it worked fine when I first bought this new phone.   I used it for a few days before I left.

Could it have something to do with the tray that the SIM card sits in?  That would also explain why the SD card no longer works.  Although I can’t think of anything that could have gone wrong when the cards were both inserted for 4 months and never touched,

You can try resetting your network settings on your phone to see if that helps. From there, select Reset Network Settings. This will erase your saved Wi-Fi networks and Bluetooth connections, so you will need to reconnect to them again.

For iPhone : https://support.apple.com/en-ca/HT201415#:~:text=Reset%20your%20Network%20Settings,that%20you%27ve%20used%20before.

For Android : https://www.androidpolice.com/reset-network-settings-android-ios/

Where did you purchase the phone? This may happen if your device is blacklisted. To verify this, you can go to the following link and put in your IMEI number. https://www.devicecheck.ca/check-status-device-canada/

I found the solution.  I had to put the phone in airplane mode for a few hours.  When I took it off airplane mode it worked again.  No idea why, must be a Samsung thing.

But thank you all for the suggestions!