Have you tried performing a hard reset? Reset your device by holding the power button and home button down (or volume down button on iPhone 7/7+) simultaneously until you see the Apple boot logo. If that doesn't work resetting all settings (Settings > General > Reset > Reset All Settings) might do the trick. If you're still experiencing problems it could also be a hardware issue with your iPhone's backlight. The good news is your 7 Plus is still under warranty unless you've done something to void it.