My Face ID stopped working and can not be set again

There may be something wrong with your TrueDepth camera. Please review the following support articles. 

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

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

If the above doesn’t help, please make a reservation at your nearest Apple Store Genius Bar or Apple Authorized Service Provider for diagnostics and service options.

Apple Store’s Genius Bar: Genius Bar Reservation and Apple Support Options - Apple

Apple Authorized Service Provider: Find Locations

Perhaps resetting your phone settings in System Settings might work. I've done that when nothing else helped.