why is my face Id not working?

Try wiping the lens.

Has an error message appeared?

If you have an error, then consider the following:

According to Apple:

Using diagnostics available in iOS 14.6 or later, Apple has detected an issue with your TrueDepth camera that might cause it to not function as expected. The TrueDepth camera contains lasers, which might be disabled for safety reasons if the device malfunctions or is damaged. This won't prevent you from using your iPhone or iPad, but until your iPhone or iPad is updated or serviced, you might not be able to use Face ID, Animoji, Memoji, and some other features that use the front camera.

As a result, I recommend service.

Service Checklist:

• iPhone service pricing.
• Check AppleCare Coverage.
• Get your iPhone ready for service.

Click ▶︎ Start a Service Request. ⚛️

Need more help?

• Chat now or call 1-800-APLCARE.
• International Support.

Click ▶︎ Check Repair Status. 🔧

Here is guidance on how to troubleshoot Face ID→If Face ID is disabled on your iPhone or iPad Pro - Apple Support. Note, however, that this is usually a hardware problem, so if the steps in the link do not resolve the problem there is a link in the article to contact Apple Support to discuss repairs.