My Face ID is not working

This is generally a hardware issue. If the unit's on an OS version older than 16.4.1, you might be able to fix it by updating it from a computer or the Software Update function in Settings; otherwise, contact Apple Support.

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Have a look at these two articles -

If an alert says Face ID has been disabled on your iPhone or iPad Pro

If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

If Face ID isn't working on your iPhone.

If Face ID isn't working on your iPhone or iPad Pro - Apple Support (AZ) If Face ID isn't working on your iPhone or iPad Pro - Apple Support (AZ)