Try turning it on in Control Center, swipe up from the bottom of the screen and tap the Wifi toggle. It will probably exhibit the same behavious as the toggle in settings but it's worth a try. If nothing works then I would suggest reloading the OS. Do a backup either in iTunes or via iCloud and then connect to iTunes and hit restore. After the reload of the OS is complete if you backed up in iCloud ignore the iTunes prompt to "Setup as new" or "Restore" and do the setup on the phone itself. If you did the backup in iTunes restore from the backup and check the Wifi. If the wifi still wont work then the next step is to reload the OS again and setup as new to see if there is something in your restore that is buggering things up.