Face ID Has Been Disabled I see warning about


  Face ID Has Been Disable
  
  A problem was detected with the TrueDepth camera. Face ID has been disabled.


in Settings.app on iPhone X. How do I fix it?
Reboot or Force Reboot

Rebooting fixed it for me. If that doesn't work take the device to Apple.

https://support.apple.com/en-us/HT208186