How do it fix my Face ID when I turn it off?

See if anything here helps 

Face ID on your iPhone or iPad Pro.  *

Face ID - If an alert says has been disabled on your iPhone or iPad Pro.  *

If Face ID isn't working on your iPhone or iPad Pro  *

Face ID not working.   -

Face ID not working' problems on iPhone.    -

Face ID Not Working.   -

You may need to take it in for service.

Genius Bar Reservation  -  US.    

Find a store.  -

Authorized Service Provider.  -

ASC YouTube for how to to schedule a repair at an Apple Store or Apple Authorized Service Provider – Apple Support

Hello ~ If working back through the steps in the article you came from doesn’t help you may need to have your iPhone checked…

Genius Bar Reservation and Apple Support Options

Also you may go to upper right hand corner and click on Get Support.

~Katana-San~