Wifi button

Try force restarting the iPhone. Hold the sleep/wake and home buttons together until you see the Apple logo and then release. Once the iPhone restarts, check. Can you turn Bluetooth on/off? If wi-fi button remains grayed out, then make a Genius Bar appointment at the nearest Apple Store to have the hardware examined. They will explain your options.

Restart and try again.

If that doesn’t work, back up using iTunes (or Finder on Catalina) to a Mac or Windows system, then reset, load current iOS, and restore your backup.

If all that fails, contact Apple Support or your preferred hardware service provider for a look at your iPhone. And you’ll hopefully also have a complete and current backup of your device now, in preparation for a potential repair or replacement.