Camera now black on iPhone 7

See this for troubleshooting your camera, If the camera on your iPhone, iPad, or iPod touch isn't working - Apple Support, including contacting Apple Support if the steps do not help.

You have to hold the Sleep/Wake and Volume Down for iPhone 7

Try closing the Camera app completely and see if it works when you re-open the app : from the home screen (i.e. not with the Camera app 'open' on-screen) double-click the home button to bring up the taskbar, then press and hold any of the apps on the taskbar for a couple of seconds or so until they start shaking, then press the '-' in the top left of the Camera app to close it, and touch any part of the screen above the taskbar so as to stop the shaking and close the taskbar.

If that doesn't work also do a reset : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content,