Why does my iPhone 4S have no service?

I've recently bought a second-hand iPhone 4S. It was on Vodafone contract and today a lady in the Vodafone shop put a pay as you go sim in the phone. As soon as the sim went in I had no service an as the lock button is broken on my phone I can turn it off with assistive touch but there's no where for getting it back on (as far as I know) so I can't turn it off to Reset it.
My question is why do I have no service?
What do I need to do and
For 10 points whether I should take the sim out and put it back in again?

Try taking out the sim and if that doesn't work try resetting it. If you can use your lock button at all try pressing that and the home button until the apple logo comes back on. If you can't do that do a reset in settings! If you still get no service, try another sim and if it still doesn't work then it looks like it is blocked.

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