So, I’ve got the exact same problem but, unfortunately the reboot didn’t help.
I will try to be clear and concise.
I’ve got an iPhone X since November 17th.
It worked perfectly.
Last night, Face ID didn’t work anymore. "A problem was detected with TrueDepth camera. Face ID has been disabled. "
Strange. I open the photo app in selfie mode. The camera works perfectly.
I rebooted the iPhone. Its didn’t help. Face ID is still disabled. Same error message.
So I turned off Face ID hoping to be able to reconfigure it afterwords. Impossible the option is grayed out.
Another reboot ...
I then noticed that there an update of iOS (11.2) is available. I install it. After rebooting, I try to configure Face ID again. The same. The True Depth camera does not seem to work.
After several other unsuccessful attempts and, I end up restoring the iPhone.
After restoration, always the same message.
I'm calling Apple support. After a remote diagnostic the verdict is without appeal: the front and rear cameras are out!
We set an appointment at the Genius Bar for a replacement of the device next week :-(
Still surprised by this diagnosis, I went back to the Photo app. the front camera was still working fine. The rear camera, however, sends me back a black screen like if the camera was covered up.
I tried to run a third-party photo application (8mm). Miracle! the camera worked perfectly.
Going further with my investigations, I got back to the Photo app. The "accelerated", « slow » and "pano" modes worked fine. On the other hand, the "video", "photo", "portrait" and "square" modes not at all!
I am still puzzled and I came to the conclusion that it is the iOS diagnostic system that prevents the Photo app and the Face ID feature from working when actually the hardware is not facing any problem.