Last night , while I was on my phone( an iPhone 4s which I have had for 3 years ) it completely froze. it does usually freeze but after pressing a couple buttons like the home button it goes back to normal or it restarts by itself. But this time when my phone froze, I tried to click some buttons etc. but nothing happened instead it just went blank, the whole screen was grey, and after a couple minutes, it went completely black. I tried everything to get it to restart but nothing happened. It's been about 9 hours and my phone is exactly the same. I've tried charging it but nothing has happened. please help.