Basically there are 2 options. Either your OS got messed up and doesn't recognize the SIM, or your phone has a hardware problem.
In the first case a factory reset should solve it, in the second case it needs to be returned for repairs (most likely a MoBo replacement).
I'd suggest trying the factory reset first. If it doesn't help, at least you can send the phone back without your personal data on it.
Here's how to do it :
With the phone turned off, press and hold the VOLUME DOWN button, and then briefly press the POWER button.
Wait for the screen with the three Android images to appear, and then release the VOLUME DOWN button..
Press VOLUME DOWN to select CLEAR STORAGE, and then press POWER.
Press VOLUME UP to start the factory reset.