Face ID stopped working

I am not aware of the ambient temperature in which you are using the iPhone. However --> Keeping iPhone, iPad, and iPod touch within acceptable ...

Store the device where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave the device in your car, because temperatures in parked cars can exceed this range.

Use iOS devices where the ambient temperature is between 0º and 35º C (32º to 95º F). Low- or high-temperature conditions might cause the device to change its behavior to regulate its temperature. Using an iOS device in very cold conditions outside of its operating range might temporarily shorten battery life and could cause the device to turn off. Battery life will return to normal when you bring the device back to higher ambient temperatures. Using an iOS device in very hot conditions can permanently shorten battery life.

Hello,

Apple has several documents helping you with Face ID.

This Support article is specific to seeing a 'disabled' Face ID alert, and explains what to do when you see that notice or an issue with the TrueDepth camera. --> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

Unless a software update solves the problem you may need to have Apple take a look at your device. Use this link to set up an appointment —> Genius Bar Reservation and Apple Support Options - Apple

If you don't see a 'disabled' message but do have general issues with Face ID not working properly, explore the list of things to try in this document: If Face ID isn't working on your iPhone or iPad Pro - Apple Support

If the first 8 don't solve the issue then use #9, contact Apple.

The message "Face ID is not available - you can use Face ID again later when iPhone cools down" typically appears when your iPhone's Face ID feature temporarily becomes unavailable due to overheating. Face ID relies on specific hardware components, including the TrueDepth camera system, to work accurately. When the internal temperature of your iPhone rises too much, the performance of these components can be affected, leading to a temporary suspension of Face ID functionality.

Overheating can occur when your iPhone is exposed to direct sunlight, used for intensive tasks like gaming or video recording for extended periods, or operated in hot and humid environments. In such situations, the device may prioritize cooling down to prevent potential damage to the internal components, including the sensitive ones used for Face ID.

To resolve the issue and restore Face ID functionality:

1. Move your iPhone to a cooler environment and away from direct sunlight.
2. Allow the device to cool down for a few minutes before attempting to use Face ID again.
3. If necessary, you can also turn off your iPhone and let it cool down completely before turning it back on.

Keep in mind that this behavior is a safety feature designed to protect your device, and it's normal for Face ID to become temporarily unavailable in extreme temperature conditions. If you continue to experience this issue regularly in normal conditions, it might be worth checking for any software updates or reaching out to Apple Support for further assistance.

If water has got in then the phone is ruined and will need to be replaced.

Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support

That happened to me and I solved it by cleaning the glass over the front facing camera, which had a smudge on it.