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 is 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?

Thanks
Every bit of help appreciated

SIM card for iphone 4s won't work?

I've put my new o2 SIM card into my new iphone 4s and the iphone says: 'this phone is not currently steup to work with carrier you're attempting to use. Please insert another SIM card from a supported carrier or request that this iphone be unlocked.' i've tried to call carphone warehouse about 6 times now and they wont pick up, and i need to have it ready by tomorrow.help?

Does iPhone's imessage work overseas?

I am going to India over the summer, and i was wondering if i can still contact a few of my friends in the US I have a 3G iphone, and the place where i am residing in India has internet.

Does imessage work overseas?

I am going to India over the summer, and i was wondering if imessage will work. In India, i will have internet. I have an iPhone 4 3G.

I already have an iPod touch but want an iPhone, is it worth it?

For Christmas last year I got the iPod touch 5th generation, and I am very happy with it, but now everyone is getting iPhones, so i was wondering if it was worth me getting one.

My contract runs out in July so I have to get a new phone then anyway, and an iPhone would probably on be 2 or 3 pounds a month more than an alternative smartphone.

I would just get a basic iPhone 4 8g version (possibly refurbished if that was cheaper), as not to waste my money, and I'm sure I would still use my iPod a lot as it has more memory and is a lot better for most things, except you can't text, call or use 3g on an iPod.

I know I shouldn't just get a phone to fit in but, being a teenager, I do feel left out not having an iPhone, and I'm sure other people do judge me for not having an iPhone.

I honestly don't even know if my parents would allow me to have an iPhone anyway, because I spent a lot of my money on my iPod recently, but if they did would it be worth it?

Thanks so much for taking the time to answer my question, and if I can I'd be happy to help you in return.

What iPhone is this and its jailbreak or no?

Hey, I have a used iphone 4 (I think) - but I'm not sure if its an iphone 4 or not
The back cover of it shows A 1332 and the model of it under "Settings" is MC608LL

Would anyone be able to confirm if this is an iphone 4 or not?

One last question: how do I know if iphone is unlocked by apple and not jailbreak?
Because it doesn't have those applications that jailbreak iphones has like Cydia etc