I need my Face ID back please

Have a look at these two articles -

If an alert says Face ID has been disabled on your iPhone or iPad Pro

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

If Face ID isn't working on your iPhone.

If Face ID isn't working on your iPhone or iPad Pro - Apple Support (AZ)

If you receive an alert that FaceID is disabled on your iPhone or iPad Pro

It can occur that you will receive the following message on your iPhone or iPad Pro : " A problem was detected with the true depth camera, FaceID is disabled"

When you're getting this error message, Make sure that you're iPhone or iPad Pro is running the latest version of iOS or iPadOS. If your device is running the latest iOS or iPadOS and you're still getting this message and FaceID is not working:  contact Apple Support to get your device serviced.