Why is Face ID not recognising my face after resetting my iPhone?

There is most likely 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. 

Genius Bar Reservation and Apple Support Options - Apple 

d_v_lm__n wrote:

my Face ID was refusing to recognize my face, so I decided to reset it, and set up Face ID again. But very frustratingly so, it still wouldn’t recognize my face there too, even when I’m in range of the square, it won’t register at all. unless I’m in a very specific spot, which makes it hard to get other angles for the next step, because it just cancels it, saying I need to reposition. And if I take too long, it just says that Face ID is unavailable. Please help, I genuinely don’t know what to do to fix this, I’ve updated my software, reset twice, to no avail. I’m so frustrated, sitting here at 4 am, crying, because I’m so frustrated and overstimulated from what should be an easy task.

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