Have you updated to the latest version of iOS? Some people have had the same problem and it was fixed after updating their device. You might check to see if an update is available. Then you can reset Face ID. I hope this helps.
EDIT: Also found this in the Apple Support forums:
Try this.
- Go to settings and turn on assistive touch.
- Go back to main settings window and click on General, scroll all the way down and click Shut Down.
- Open up assistive touch by tapping on the circle, once it’s up press on hold on the circle until you see your screen flicker which means that you have cleared your cache.
- Perform a hard boot by pressing up, down volume and on button simultaneously.
- Once you completed all the steps you should be able to reset your Face ID.
This is what you will have on your screen once you got back on the General setting and had tapped on the assistive touch button. Then press and hold on the button above “HOME”:
View attachment 949199