Samsung Galaxy S II :: Not Making Calls, Texts Or Accessing Data Network

So basically I went away for 4 weeks, came back, switched on my phone and now BOOM it won't call anyone, it won't use 3G or anything or even make texts.

Here's some useful information:

- I'm picking up signal, and can receive texts etc.

- when I try and call someone, it says 'you cannot make outgoing calls while call barring is on' (even though I've checked in settings and it isn't).

- I'm on o2 in the UK

- I've checked my status, and it says 'Mobile Network State' is 'Disconnected'.

- It's running 2.3.3

- I've got the Network Mode on Auto (as it always was when the phone actually worked), and also I haven't been playing around with the APNs or anything like that.

- I bought the phone legit from Carphone warehouse a few months ago, and it worked mostly fine up until I switched it back on yesterday. I'd had a few problems with non-wifi internet before I left last month, but nothing affecting making calls etc.