why is my Face ID not working?

Try these steps:

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

The support article you came here from should provide you with the steps needed to try to resolve this issue. Have you tried them all yet? If so, and they didn't help, I suggest that you contact Apple Support directly. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page.