How can I enable my disabled Face ID?

This Support article is specific to seeing an ‘issue’ or '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 an issue 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 and Service - Apple Support

Without full details it's difficult to offer suggestions, but perhaps this will help:

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

Regards.