IPhone 4 on Three network has no service?

My iPhone 4 on the Three (3) network hasn't been able to get a bar to call anyone or text since yesterday. My mum is on 3 too and is having no problems.

Can someone help? Do I need to go to the apple shop or 3 shop? What's wrong?

The three network has more internet "crashes" than a Motorway pile-up!