Facial recognition has stopped working

Try the help article:

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

Reset Face ID

• If you still need help, reset Face ID, then set it up again. Go to Settings > Face ID & Passcode and tap Reset Face ID. Then tap Set up Face ID to set it up again.
• If you can’t enroll your face, take your device to an Apple Retail Store or Apple Authorized Service Provider or contact Apple Support.

Another:

https://www.macworld.co.uk/how-to/iphone/fix-face-id-not-working-3690315/

Try restarting your iPhone until the Apple logo appears

Restart your iPhone - Apple Support

Hello,

How does a HomePod recognize one's face?

Did you mean voice recognition? 😀

Or is this about your iPhone or other device?

*meant to reply to the OP