The SIM card slot is located at the bottom of the unit above the serial number label. The SIM card pinhole ejection button is located to the right of the SIM slot:
Press the button with a pin until the SIM tray pops out:
Take the SIM card out of its tray gently and give it a quick clean by wiping with an alcohol swab. Or you can wipe it with a clean cloth if you don't have an alcohol swab. Then put it back in the right way.
Power cycle the unit after putting the SIM back in. The article How to Power Down an Observant C3 has instructions on how to power down a C3. Once the unit is off give it a minute and turn it back on again by pressing the power button.