You seem to be using an iPhone?
If so iPhones can require activation on some networks due to the stupid way that Apple do things.
If this is the case then try putting some cash credit on your account, switch off and on again and wait a while and see if it starts working.
The reason for the cash credit is that iPhone activation sends a UK SMS which costs 25c on 48 and won't go unless you have cash credit on your account to pay for it.
Make sure that roaming/data roaming is also enabled.
Hope this helps.
