If my Face ID was not working anymore what can’t I do to make it works back

If you're sure that nothing (cover, screen protector, dirt) interferes with your front camera, and if the issue persists (after performing the steps provided in the support article that led you here), contact Apple Support directly, using this specific link:--> Apple Support , or make an appointment and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio