Put your iPhone in Airplane Mode
Often simply switching cellular data off and then on again will refresh your phone and help it to recognize the SIM card again. There are several ways to shut off data on your iPhone, but the fastest way is by using the iPhone's Airplane Mode.
1. Once your iPhone is unlocked, swipe up from the bottom of the screen.
2. In the upper-left corner tap the Airplane Mode icon that looks like a circle with a white airplane in the middle.
3. Tap the Airplane Mode icon again to turn data back on. When Airplane Mode is on, this icon will turn orange. When it's off, it will have a transparent background.