Is there a way to turn your Face ID back on? My has disabled

This is generally a hardware issue. If the unit's on an OS version older than 16.5.1, 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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Follow the guidance in the Related Article that you were reading when you posted, including the instruction to contact Apple support if the other steps don’t resolve the problem.