I agree with Nigel, it's almost certainly a dead screen. It could be the backlight on the screen, but that's a moot point since the cure for that is also replacing the screen. Here's the guide you'll need to do the job right.
[guide|67563|iPhone 7 Plus Screen Replacement - iFixit Repair Guide|new_window=true]
As outlined in the guide, be very careful when moving the home button to the new screen; if it gets damaged it can't be replaced and still have the Touch ID work except by Apple; you'll have to play them full price for a screen replacement to get it working again. The iPhone 7 plus is old enough now that the price for parts has dropped drastically; you can find a screen for under $20 USD on sites like Amazon, eBay and AliExpress.