I have had my Samsung Galaxy S5 for 6 months and it has worked fine. Just yesterday, I noticed the "no service" notification and there is a note at the bottom of the screen that says "unregistered SIM". I have not changed anything in the last few days. How did this happen and how do I fix it?
Your SIM card connections might have been compromised. It happens sometime. Take out the battery the SIM card and re assemble. May switch phone to airplane mode and back. If that doesn't help you may need another SIM card.