I had this same problem and I think it, for me, it was due to moisture. Let me explain...
I just got back from a bike ride and I was very sweaty. I got a phone call and after there was sweat around the ear speaker where I was listening on the call. Shortly after that I got the message saying the fingerprint sensor was not available. Shortly after, it was no longer in the settings.
Fix that worked for me...
I put the phone in a sealed bag with rice for a few hours and it corrected the issue.
So, if you bring your phone in the bathroom when you shower, this could be your issue.
I hope this helps