I can't turn on my tablet - Apple iPad (3rd generation) iOS 5
Cause 4 of 10: Your tablet is locked to another operator
If your tablet is locked to another operator and you're trying to use a Vodafone SIM, the display will tell that your SIM is invalid.
Solution: Before you can use a Vodafone SIM, you need to unlock your tablet using an unlock code. To get the unlock code, contact the operator that locked your tablet.