how can I fix my Face ID?

This Support article is specific to seeing a 'disabled' Face ID alert, and explains what to do when you see that notice or an issue with the TrueDepth camera. --> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

Unless a software update solves the problem you may need to have Apple take a look at your device. Use this link to set up an appointment —> Genius Bar Reservation and Apple Support Options - Apple and this one to get a repair estimate —> iPhone Repair & Service - Apple Support

If your Face ID is disabled there is most likely something wrong with the hardware. Please update your iPhone to the latest avaliable version and if this is not working, please Contact - Official Apple Support