my face id is not working on my iphone what do i do

See this support article:--> If Face ID isn't working on your iPhone or iPad Pro - Apple Support

and/or this one:--> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

If, after performing all the steps provided in the above support articles, 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 - Find Locations - Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio