I've seen this two different ways. One, the connector isn't seated well at the home button, disconnect/reconnect and retry.
The other way I couldn't figure out how to solve it so I backed it up, unplugged the home button and did an itunes update/restore so it would throw an error code. I then buttoned up the iphone so it looked like I had never been in it, carried the bricked iphone to apple and said "I dunno what happened, I did an update and it did this" they replaced it under warranty. **THIS WILL BRICK YOUR PHONE AND NO GUARANTEE APPLE WILL REPLACE IT**
On the same note, if you cannot solve this problem, I suggest you not ever update the phone. touch ID or home button issues seem to like to brick iphone 6's