Face ID suddenly stopped working on my iPhone

According to Apple:

Using diagnostics available in iOS 14.6 or later, Apple has detected an issue with your TrueDepth camera that might cause it to not function as expected. The TrueDepth camera contains lasers, which might be disabled for safety reasons if the device malfunctions or is damaged. This won't prevent you from using your iPhone or iPad, but until your iPhone or iPad is updated or serviced, you might not be able to use Face ID, Animoji, Memoji, and some other features that use the front camera.

As a result, I recommend service.

Service Checklist:

• iPhone service pricing.
• Check AppleCare Coverage.
• Get your iPhone ready for service.

Need more help?

• Call 1-800-APLCARE.
• Contact Support.
• iPhone Support.
• iPhone User Guide.

Click ▶︎ Check Repair Status. 🔧

Good luck! 👋🏼😉

If Face ID doesn’t work and Stolen Device Protection is enabled your only options are to get Face ID fixed or follow this guidance→How to remove Stolen Device Protection af… - Apple Community

Was Face ID working before you updated?

Face ID is hardware; if it no longer works it can’t be fixed with software. Resetting the phone will not fix broken Face ID, but it will allow you disable SDP.

For Face ID failure, here is Apple’s support→If Face ID is disabled on your iPhone or iPad Pro - Apple Support; there is some limited troubleshooting you can do, but it usually requires a hardware repair. There’s a link in the support article for contacting Support.