WiFi and Bluetooth are grey

Reset Network Settings: Only network settings are removed.

• When you reset network settings, previously used networks and VPN settings that weren’t installed by a configuration profile or mobile device management (MDM) are removed. Wi-Fi is turned off and then back on, disconnecting you from any network you’re on. The Wi-Fi and Ask to Join Networks settings remain turned on.
• To remove VPN settings installed by a configuration profile, go to Settings > General > Profiles & Device Management, select the configuration profile, then tap Remove Profile. This also removes other settings and accounts provided by the profile. See Install or remove configuration profiles on iPhone in this guide.
• To remove network settings installed by MDM, go to Settings > General > Profiles & Device Management, select the management, then tap Remove Management. This also removes other settings and certificates provided by MDM. See “Mobile device management (MDM)” in the iOS Deployment Reference.
• Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.

If the above fails....

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

Sypintermt Said:

"WiFi and Bluetooth will not turn on"

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If Wi-Fi and Bluetooth are Greyed-Out:

Try The Following in the Order Provided:

A. Restarting the iPhone:

If any changes were made (i.e. Installed Apps), then Settings (i.e Wf-Fi), may have become manipulated technically. So, try restarting the iPhone:

1. Go To: Settings
2. Tap: General
3. Tap: Shut Down
4. Slide: Your Finger to the Right

B. Disable Cellular:

Try disabling Cellular, and using Wi-Fi instead, seeing if that makes all the difference. Cellular always overrules Wi-Fi.

C. Try manually restarting your iPhone:

1. Go To: Settings
2. Select: General
3. Scroll to: the Very Bottom
4. Select: Shut Down
5. Slide Finger: to the Right
6. Wait: 30 Seconds
7. Power One: the iPhone
8. Try this Once More

As a Last Resort:

E. Restore form a Backup:

Have a backup of your iPhone? If you are not concerned with losing data, then restore your iPhone from a backup that was made when Wi-Fi was working. Prior to restoring, backup your iPhone, incase anything goes wrong with the restoration.