Face ID disabled, how to fix it

Start here:--> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

If the issue persists, 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