If you are having issues with your Samsung S6 or S6 Edge Plus or Note 4/Note 5 having issues detecting your Sim Card, follow these steps.
1. Turn off your Samsung Galaxy S6 and wait about 20 seconds. Start up your phone again.
2. Removing the Sim card and blowing air in may be very helpful in fixing this issue too.
3. Check for any Sim Card damage. If Gold area is damaged, you may need to get a new Sim from your carrier, usually your carrier will give you a free Sim.
4. You can also test another Sim card to rule out if it is a Sim card issue or a phone issue. Usually step 1 fixes the issue.
5. Another thing you can do is power drain. Remove the battery and press the power button 10 times to power drain it while it is disconnected from battery and also USB charger. You would be surprised at how much stuff gets fixed by simply battery draining.