Why does the WiFi grayed out and wont turn on?

Settings > General > Reset > Reset All Settings

Try a Reset All Settings.

Go to Settings > General > Reset > Reset All Settings

• What this effectively does is to reset some of your Settings to factory default. It’s not a complete removal of settings. It removes all of your preferences for Wi-Fi, Bluetooth, Do No Disturb, Notifications, General, Sounds, Brightness & Wallpaper and Privacy.

• Resetting all settings also reverts all your personalized and customized features like wallpapers and accessibility settings back to factory defaults.
  • Performing this action does not impact any of your personal device data, including photos, videos, notes, messages, reminders, emails, calendars, texts, documents, and other files

See the Apple recommended response to a similar thread.

https://discussions.apple.com/thread/8506538?language=en