IPhone needs to "warm up"

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.

Hi, you can check this link: If Face ID isn't working on your iPhone or iPad Pro

Download the latest iOS software update, go to Settings > General > Software Update then setup Face ID again.

If the issue persists, you can restore your phone as new:

How to back up your iPhone, iPad, and iPod touch

Restore your iPhone, iPad, or iPod to factory settings

Otherwise, you can take your phone to an Apple Store or Apple Authorized Service provider. You can book an appointment on getsupport.apple.com or use the Apple Support app.