I want to get faceID back

This is generally a hardware issue. If the unit's on an OS version older than 16.7.1, you might be able to fix it by updating it from a computer or the Software Update function in Settings; otherwise, contact Apple Support.

(245670)

ker194 Said:

"I want to get faceID back: My phone is stuck face id, please help me"

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Troubleshooting Being Stuck in a Boot Loop:

Your iPhone might stuck in a boot loop. This typically happens when you unsuccessfully download something or an install stalls while in the process.  So, refer to my User Tip: How to Fix an iPhone in a Boot Loop - Apple Support