How to get back my face id?

Face ID or Apple ID?

On what iPhone model? Which iOS is currently running on it?

Face ID:

If you're sure that nothing (case/cover, screen protector, dirt) interferes with your front camera (remove any obstacles, if present), see this support article:--> If Face ID isn't working on your iPhone or iPad Pro and/or this one:--> If an alert says Face ID has been disabled on your iPhone or iPad Pro

If the issue persists, after performing all the steps provided in the above support articles, 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:--> https://locate.apple.com/choose-country-region. After checking it out, they'll give you all the available options.

Apple ID:

See these circumstances:

1) Apple ID disabled (generic) = If your Apple ID is locked or disabled - Apple Support

2) Apple ID disabled (in iTunes & App Store) = Contact Apple Support, after reading the steps provided in this support article:--> If you see a message that says 'Your account has been disabled in the App Store and iTunes' , and using the following specific link:--> Apple Support

If you get no help, contact Apple Support using this link:--> Contact Apple for help with Apple ID account security - Apple Support

Regards

Giulio