What do I need to return my iPhone to Apple?

Ok, so I live in the UK and I've had my iPhone 4S for around a week. Today, I went to take it off charge to check the weather, as I always do. But I pressed the lock button and nothing happened. No lock screen. The iPhone was just blank. A black screen. I tried charging my iPhone again and nothing happened. I changed my iPad with the same charger and it worked perfectly. I tried restoring my iPhone with my mac but it couldn't pick up on the iPhone.

So I'm with an iPhone 4S that doesn't turn on.

I have a few questions.
One is will I be able to get a refund from the apple store even though I can't find the recipt (yes it's only been a week but I'm really bad at stuff like that). I did ask for a printed copy because I rarely use my email and I knew I'd never be able to remember the password but I've actually managed to lose the printed copy. So is that going to be a problem?

If it is, then what are my options?
If its not, I guess I should take my iPhone in right away.

Have you tried holding the power and home buttons simultaneously for 10 seconds?

If that doesn't work and your Mac doesn't pick it up, and its only been a week since you got the iPhone, go to apple.co.uk and book an appointment with an Apple Genius and let them take a look at the iPhone for you… They should replace it…

You must always have a receipt with your product. It is required as evidence so the shop know you didn't steal it. Yes, it will be a problem. You can take it to an Apple Genius and have them look at your problem and solve it.

Take it to the apple store, they will either fix it or just swap it with a new one since you just bought it.