How do I fix Face ID not working on my iPhone?

When Face ID doesn’t work, it either means there is a software bug or something obstructing the TrueDepth camera, or a hardware issue with it. Please first review the following support articles to determine if this is a software issue or something obstructing it. 

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, this indicative of a hardware issue in which you should make a reservation at your nearest Apple Store Genius Bar or Apple Authorized Service Provider for service. 

Genius Bar Reservation and Apple Support Options - Apple 

See this article.

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

Go in settings and reset your Face ID