IPhone 4 says restricted network?

Normally I have signal everywhere I go but I got back from school today and my phone has had no service since then. I've tried turning my phone off and back on again but still no signal, I've gone into settings and tried everything I can but a message keeps coming up saying "Your selected mobile network is restricted. You can choose a different network in Settings." I've tried selecting different networks and clicked back onto O2 (my network) and still nothing.

Get with the now and buy a Nexus 4