I changed my screen and my Face ID is not working

Have you had it changed at apple or at an authorized service center? If so, go back to Apple or to the service center to discuss your options.

Have you had it replaced at a non authorized service provider? Then most of the times there are no original parts used, and that means that Face ID will not work anymore I am afraid

Do you see a genuine parts error message?

A message about a non-genuine part is discussed in this support document: iPhone parts and service history - Apple Support

An Unknown Part message appears if the part installation is incomplete or if the part:

- Was replaced with a nongenuine part (e.g., you had it repaired with a third party service that did not use an Apple component)

- Was already used or installed in another iPhone

- Isn't functioning as expected