How do I fix the TrueDepth camera on my iPhone?

This Apple Support article is specific to seeing an ‘issue’ or '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 an issue 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 and this one to get a repair estimate —> iPhone Repair and Service - Apple Support

You make an appointment at the Genius Bar of your local Apple Store or take it to your local authorized service provider.

Note that if the device has been serviced by an unauthorized party, for example a 3rd party shop replaced the battery or screen, your phone is now untouchable by Apple or their agents.