Power your device off, remove the power cord and battery. Take out the SIM card and put it back again.

Some routers have a metal cover that clicks in to hold a SIM down, try adding a small cut out piece of paper to add resistance and help the connection.

Make sure the SIM card is sized properly for the device (or use an adapter) and is facing the correct way to make electrical contact. 

It could also mean your SIM card has become damaged, but this is pretty rare.