I can't connect to App Store after iOS 7 upgrade few weeks?

I'm having issue on my iPhone 5.iOS 7.03
I have tried everything as ppl mentioned.
Check the time and date, and I also tried to sign in from App Store and from setting as well.
But it won't work for me!
Can someone please help me out.

Added (1). Can't connect iTunes Store

This morning I deleted WhatsApp from my iPhone, now I can't redownload it?

So I deleted my account & the app but now I downloaded the app again. It asks for my phone number but when after I type it & click "Done" an error message comes up saying "Could not connect to WhatsApp Service. Please try again later."
I already tried deleting and redownloading the app a few times.
I tried rebooting my phone.
Is it because I have to use a different number than the one I was using before?

IPhone 4S, iOS 7 to iOS 6 downgrade?

Any possible way to downgrade iOS 7 to iOS 6 on iPhone 4S without having to jailbreak? Please help

Thank you for your time.

Added (1). "Factory default" via iTunes maybe?

Does using iphone 3g cause a lot of data?

I have a question about my celular data. I can only use my celular 350mb for 1 month.
does using 3g affect my data faster? Its already 45mb.i've only use it for an hour.
should i turn off my 3g?
i've notice that watching youtube cause a lot of data. But i decide not to watch youtube videos. Should i turn off 3g?