How can I get my Face ID back please

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You need to mention which part of the article was not clear to you.

If you still need help, reset Face ID, then set it up again. Go to Settings, tap Face ID & Passcode, then tap Reset Face ID. Next tap Set up Face ID to set it up again.

If Face ID isn't working on your iPhone or iPad Pro

If your iPhone has a history of Camera replacement / Repair / Service from an un-authorised service provider then the only option that you have is to take the device to...

Take your device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support:

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Contact Apple for support and service - Apple Support 

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Contact Genius Bar.

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Hello,

Apple has several documents helping you with Face ID.

How to set up and use Face ID is described in this support article --> Use Face ID on your iPhone or iPad Pro - Apple Support

- including how to turn on

This document is specific to seeing a 'disabled' Face ID alert, and explains what to do when you see that notice. --> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

If you don't see a 'disabled' message but do have general issues with Face ID not working properly, explore the list of things to try in this document: If Face ID isn't working on your iPhone or iPad Pro - Apple Support

If the first 8 don't solve the issue then use #9, contact Apple.