Welcome to iMore! Have you tried soft resetting of the device? To perform a soft reset, hold both the power button and home button down until you see the Apple boot logo.
You may also want to try contacting your carrier to make sure that the SIM hasn't become deactivated.
If that doesn't fix the problem try
erasing your device and setting it back up. I recommend setting it up as a new device as it provides the cleanest possible installation.
Lastly, your phone is still under warranty assuming you haven't done anything to it that would void it. If you're still experiencing issues, take it to an Apple Store or contact
Apple Support. If your problem is hardware related they will exchange it for you.
If you're still experiencing issues let us know!
