iPhone 6s will not start after failed update to current iOS

I think a family iPhone 6s was not plugged during an update to the newest iOS. Noting will power it up now-just a black screen.

I plugged it into iTunes which says the discovered iPhone needs to be restored and updated. While doing that I get the white apple and a progress bar that never moves. After awhile iTunes reports:

The iPhone "iPhone" could not be restored. An unknown error occurred (4).

I've restarted and computer and tried on a couple of computers