Hi! Try to reset your device. This won't delete your data stored on this device.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
Alternatively, you may go to Settings - General - Reset - Reset All Settings
https://support.apple.com/en-us/HT201559
Try the recovery mode. Turn your device off, then plug it to your computer with the home button pressed. Keep the Home button pressed until you see the iTunes logo on the screen of your device. After that on your computer you should see the iTunes window saying your device needs to be restored to factory settings. Click on the restore button.
More info here:
https://support.apple.com/en-us/HT201263
Try using the DFU mode:
- Connect your device to your computer using a USB cable.
- Turn off the device.
- Press and hold the Power button for 3 seconds
- Press the Home button while holding the Power button and hold both for 10 seconds.
- Release the Power button, but KEEP HOLDING the Home button.
- After about 15 seconds, iTunes will tell you it has detected a device in Recovery Mode.
- Restore your device
More info here: https://www.theiphonewiki.com/wiki/DFU_Mode