My iPhone 7plus camera isn’t working

1: Close the Camera App

Sometimes the camera app on your iPhone does not get loaded properly, which causes the camera black screen problem. In that case, fix the issue by closing the app of the camera forcefully. To operate such a function, get the preview of the app by tapping the Home button twice. Now, swipe up the interface of the camera and close the cam-app. After doing that, wait for 5 minutes and restart your phone again. This may help to solve the issue.

2: Switch the Front or Rear of the Camera

This trick may help you to deal with the black screen problem without going to an iPhone repair workshop. Sometimes the users observe the front camera of the iPhone is not working well. In this situation, switch to the rear-cam by tapping on the camera icon, and after a few moments when you turn on the front camera, you may find the black screen issue is gone.

3: Switch off the ‘Voice-Over’ Feature

It sounds surprising that the black screen issue is observed when Voice-Over feature is on. You can simply resolve this issue by following some steps. Go to the phone Setting>General>Accessibility and turn off the ‘Voice-Over’ feature. After that, wait for a while and again launch the camera app.

4: Restart the iPhone

The common way to fix iPhone black screen camera issue is to reset the power cycle of the device by pressing the Power (Wake/Sleep) button of the device for a few seconds. This will help to display the Power Slider on the screen of the iPhone. After that slide and turn off the phone and wait for 30 seconds before pressing the Power button to turn your device on.

5: Update the iOS Version

There are high chances of a black screen issue on the iPhone due to the outdated and unstable version of iOS. To fix the problem by updating the iOS version in the mobile device. Unlock the phone and go to the Setting>General>Software Update. You may find the latest version of iOS here and tap on the ‘Update & Download’ or ‘Install Now’ button to update the iOS device to a stable version. However, you need to keep in mind that your phone is charged at least 60% before you start the procedure.

Final Thoughts

Few other things you can do to fix the issue are:

• Try switching between various video apps like FaceTime or Snapchat
• Reset your iPhone to factory default settings – Go to Settings > General > Reset > Reset All Settings.
• If resetting to factory default settings does not work, you can try erasing all data and reset the phone. Make sure to back up your data as this will delete all the data from your iPhone – To erase all data from your iPhone, Settings > General > Reset > Erase All Content and Settings.

If the Flash on your iPhone is out of sync, it can cause the camera not to work also. Try the below steps:

1. Remove your iPhone from the case.
2. Clean flash and lens.
3. Tap the lightning bolt in the upper left of the screen in the camera app and make sure the flash is On.
4. Restart your iPhone (press and hold the Home and Power/Sleep buttons).
5. Turn on/off the Flashlight from the Control Center. 
6. Turn off Camera location services. Go to Settings > Privacy > Location services and switch it off on the Camera app.

(copy and pasted from: Five Technical Steps to Solve the Black Screen Problem in iPhone Camera)

I hope it helps.