When you connect your iOS device to your computer, iTunes should automatically recognize it and start syncing. However, sometimes nothing happens. Here are some possible solutions:
- Try connecting to a different USB port: The USB port may simply not be working. Try plugging your device into different ports to see if iTunes recognizes your device.
- Check every connection: Make sure the cable is fully plugged in to your device and into the computer.
- Restart your device: Press and hold down the Sleep/Wake button until the device shuts down; press and hold down the Sleep/Wake button to turn it back on; try connecting again.
- Restart your computer: Restart your Mac or PC computer and try connecting again.
Device Firmware Upgrade (DFU) mode bypasses the current version of iOS installed on your device and lets you restore your device. It's recommended if you are having problems using the iTunes Restore option. 
The iTunes Restore feature reinstalls the latest version of the iOS, restores all settings to their defaults, and deletes all music, apps, videos, and other data on your device. However, if you've backed up your device, you can restore that backup (including all data and settings) after you've restored the iOS. Restoring your device is recommended if you're still having problems after restoring settings and have restarted or reset your device. Note that you must be able to connect your iOS device to your computer to use the Restore feature. 
If your device keeps shutting off without apparent reason, or if it is functioning in strange ways, your device settings may have become corrupted. You'll need to restore them to the original settings. This will not erase your contacts or calendar data, or delete any music or videos that you have downloaded to the device. 