I am having the same problem on my iPhone XS. This problem started approximately 2 days ago. I tried to reset the FaceID but device said "FaceID is not available". Then I restarted my iPhone, tried resetting FaceID again, managed to made the reset.
After a couple of minutes or hours, not important, FaceID stopped working again. Reset was not possible, again. Restarting device makes the reset possible, again.
When FaceID stopped working, I tried to take a selfie portrait with front camera, I found out it could not focus on my face. I think this is a software bug in the drivers or softwares that manage the true depth camera. If my true depth camera was broken or had hardware problems, it should not work after device reset.
I read many replies below this post, people are complaining about similar issues, I think this problem is not the FaceID problem, it is a software/driver related true depth camera problem of the front camera.
I do hope this issue is fixed in a near future.