Hi
I had the same problem my iPhone 4S. I tried reseting the device, turning it of and pressing all buttons without any result. I opened the back cover and tried the method explained in the youtube video, but sadly it didn't work. When I was about to give up I desperatly hit the phone in the table, and dropped it from 2-3 cm height. Then I hold the home and on button for 20s, and to my surprise it worked!!!
So it looks like a loose connector was the problem in my case. I'm not sure how long it will last, but I'm happy for now.
Good luck!