My Face ID stop working

(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2) If that does not help your phone might need service ➔  Contact Apple for support and service - Apple Support and take advantage of the Apple 1YR Limited Warranty which should still be active on your iPhone 12 Pro Max.

Axel F.

Glad it worked out for you.

Axel F.