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The cable to iTunes symbol indicates that the firmware/software of your iPhone
is corrupted. Your only option is to force-restore the iPhone to factory settings through iTunes application (from computer).
If you've already connected to iTunes, then the restore process would start automatically and it would take couple of hours to complete the process. You should see the progress on iTunes. Do you see it ? How much time is remaning ?
If it is not responding, then follow the steps below to restore your iPhone by putting it to DFU Mode.
First download and install latest iTunes on your computer from here (ignore this step if you already have latest iTunes) : https://www.apple.com/itunes/download/
Then connect the iPhone to your computer with the USB Cable.
Open iTunes application on your computer.
On your iPhone press and hold the Home button and the Power button at the same time.
After EXACTLY 8 seconds release the Power button. Continue holding the home button until your iTunes shows a message telling you that it has detected an iPhone in recovery mode.
Click OK or Restore
Note : Put it to DFU Mode while connected to computer and also in DFU Mode your iPhone will remain blank and will be unresponsive. It is normal. Timing is critical when you put it to DFU Mode. So keep trying until it goes to DFU Mode. And it will take a while to restore it.
Please let me know if that didn't work, so I will continue to help you until we fix the problem