There's probably nothing you can do yourself. If you have another phone unlocked/locked to O2 then try putting your SIM in it and see if that one works fine. If it doesn't then as others have said, there is probably an outage in your area. If the other phone works fine, you will need to get your phone looked at.
the coverage is fine it says. plus i live with my mum who has o2 and she would have said something.
i have around £9 credit so it's not that. but even if i didn't have credit, it shouldn't say this. the words "limited service" are in the middle of my screen.