If it shows this icon on the status bar, it means that it does not detect the SIM card.
a). Check the installation of the SIM card.
Check whether the SIM card is installed well or not. Try to install the SIM card again.
b). Check the direction of the SIM card.
Check whether the installation direction of the SIM card is correct or not. If not, install again.
c). Check the SIM CARD slot.
Check whether the SIM CARD slot is normal or not.
d). Scratch the metal spring of the SIM card slot.
Maybe the surface of the metal spring becomes oxidation. Try to scratch the metal spring of the SIM card slot with emery cloth or knife.