try to restore it in (DFU) mode by following these steps:
*before that If you see the battery charge warning, let the device charge at least for 15 minutes
1. disconnect usb cable from the iPhone, but leave the
other end connected to your computer.
2. turn off your iPhone by pressing and hold the power button for a few seconds
until the red slider appears, then slide the slider. wait for the device to turn off.
If you cannot turn off the device using the slider, press and hold the
power button and home buttons at the same time. when the device turns off,
release both buttons.
3. while pressing and holding the Home button, reconnect the usb cable to the
device. The device should turn on.
4. keep holding the home button until you see the "Connect to iTunes" screen.
as soon as this screen show up release the home button.
6. then use itunes to restore the device.