A reboot of the computer, much as when other computer applications suddenly misbehave or become unresponsive, is almost always required for them to operate regularly again. Simply switch off your computer for a few seconds before turning it back on as usual. If your computer is freezing or not responding, simply press and hold the power button until it turns off. Unplug the power cord from the power source if this is the case. For a few seconds, keep it switched off.s then power it back on.

Before rebooting the computer, make sure your iPhone 7 Plus is unplugged from the USB port.

Restart your iPhone once you've rebooted your PC (soft reset).

To do so, hold the Power (Sleep/Wake) button down for a few seconds until the Slide to Power Off screen appears. To totally turn off your gadget, drag the slider from left to right. Press and hold the Power button for another 30 seconds, or until the Apple logo displays. After that, your iPhone will reset.

Connect your iPhone to the computer again after the restart, then try to backup to iTunes to see if it works.