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
If this does not help, restore your device to factory settings. Note that this will erase the data from your device.
Take a look this Apple Support article: Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.
As a final step, try 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
If these steps don't work, contact Apple and ask for help.
https://www.apple.com/support/iphone/contact/
