What to do when Face ID stops working on iPhone?

Usually that is a hardware problem. Follow this guidance from Apple→If Face ID is disabled on your iPhone or iPad Pro - Apple Support, including contacting Apple Support if none of the troubleshooting steps work.

Zbobbity said:

Face ID just stopped working and I’m unable to reset due to no reset button to assist. Any thoughts?

Usually, when this happens it means that there is a problem with the True Depth Camera and the IR sensors may have been damaged.

If you need more information about this then please visit the following Apple community support pages:

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

I hope this helped you, otherwise, please ask again if you need more help!