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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
iPhone Camera
With iPhone 12 models and later using iOS 15.2 and later, you can go to Settings > General > About to see the parts and service history of your iPhone and you can see if the camera has been replaced. If the repair was done using genuine Apple parts and processes, you will see "Genuine Apple Part" next to Camera.
Parts and service history
With iOS 15.2 and later and iPhone XR, iPhone XS models, and later—including iPhone SE (2nd and 3rd generation)—you can go to Settings > General > About for the parts and service history of your iPhone. You can also find out if the battery has been replaced. If the replacement was done using genuine Apple parts and processes, "Genuine Apple Part" will appear next to Battery.
Do you have a disabled Face ID alert on your device?
Please read the "What to do Next" section in this link, if nothing works as per the instruction given in this link --> If an alert says Face ID has been disabled on your iPhone or iPad Pro
