Hi
Turn iPhone off Make sure your pc is running the latest iTunes 12.7
Connect lead to USB on your pc
Press & hold just the power button down on iPhone don't let go
Connect lead to iPhone keep power button pressed down until pc sees iPhone Then wait for restore you may need to do this more than once Before it works.
If you still have a problem go to Support.apple.com
Cheers
Brian 🇬-1F1E7;