can you give us more details? what happened? is this a new phone? did you get a new sim card? who is your carrier?

just remember the more details you give us the better we can help.

 

1: Restart Your Android Phone
2: Check SIM Card Tray
3: Insert Valid SIM Card
4: Toggle Airplane Mode
5: Reinsert SIM Card
6: Select Correct Network Operator
7: Buy A New SIM Card
8: Set Network Mode to Auto
9: Clear Cache To Fix Invalid SIM Card Error
10: Use Phone In Safe Mode
11: Do Factory Reset Of Your Android Phone
Try from 1 to 11