iPhone 4 won't turn on

[liamox]

Hi. My brothers iPhone 4 battery died (1% and turned off). He put it on charge and everything and it just won't turn on.
We have also replaced the battery and still won't turn on.
Connected it to the computer and nothing. Held down the power button and middle button and left go to enter dfu mode.. The computer then finds it because it installs the drivers (screen still blank and no power). iTunes won't pick it up in dfu mode.. What's happened to it?
It's also jail broken but has been for ages and no problems

 

[Just_Me_D]

I'd make a Genius Bar appointment with the nearest Apple Store and have them troubleshoot it since you've tried everything at your disposal.

 

[natasftw]

If I had to guess, it's the actual dock that's gone bad. You've replaced the battery. The other things you can easily troubleshoot yourself are checking the cable by using different cables and checking the USB port by using different ports/computers.

If none of those work, the dock is the next place to look. For the iphone4, a third party iphone repair shop would be considerably cheaper than the genius bar.