Mobile phone jammed

Question

One day and all of a sudden, when my daughter switched her mobile phone on there's a message that states that her security code was invalid. She has had "3" mobile as a provider since the phone was bought and she's on a pre-paid plan. I have been in a battle with their "customer care" people for 3 months but they're jerking me around. Any ideas? The model is an Lg U8120.