For this problem, you only need to do a few troubleshooting procedures and don’t worry, they’re pretty easy and safe.

1. Force restart your phone

   Before anything else, make sure that this problem isn’t due to a minor system glitch. That’s why you need to do the forced restart first to refresh your phone’s memory and reload all apps and services. 
   
   1. To do this, press and hold the volume down button and the power key for 10 seconds. This will force your phone to power itself down and turn back on. 
   2. When the logo shows, release both keys and wait until the device has finished rebooting. 
   
   After doing it, open Messenger and see if the camera is now working. If still not, then move on to the next solution.
   

2. Check the permissions

   When an app like the Messenger tries to use the camera, more often than not, you will be prompted about it and you will choose whether to allow or deny it. However, there are times when the prompt wouldn’t just pop up. That’s the reason why before we proceed, it’s best to check the permissions of the app. 
   
   1. Swipe up from the bottom of the screen to pull up the app drawer. 
   2. Go to the screen where the Messenger app is located. 
   3. Tap and hold on its icon until options show. 
   4. Tap App info. 
   5. Scroll down to find Permissions and tap on it. 
   6. The next screen will show you the services that the app is allowed to access. 
   7. Make necessary changes and then try using Messenger again. 
   
   After making sure that the app is allowed to use the camera and yet it doesn’t work during video calls, then move on to the next solution.
   

3. Test the camera

   For this procedure, all you need to do is test if the camera works on its own. We just need to rule out the possibility that the problem is with the camera of your phone. 
   
   To do so, launch the camera and try to snap pictures or record short videos. If the camera works properly, then move on to the next solution.
   

4. Clear Messenger’s cache and data

   Doing this will reset the app. This is a very effective solution to app problems and at this point, we can already assume that the problem is with the Messenger itself. So try to follow these steps to clear its cache and data: 
   
   1. Go to the screen where the Messenger icon is located. 
   2. Tap and hold on the Messenger icon until options show. 
   3. Tap App info. 
   4. Touch Storage. 
   5. Tap Clear cache. 
   6. Now, tap Clear data and touch OK to confirm. 
   
   After resetting the app, open it and try to see if the camera works.
   

5. Uninstall Messenger and reinstall it

   After doing the previous procedures and the camera is still not working on Messenger, then it’s time you uninstalled the app and reinstalled it. Uninstalling it will remove all its files and data that may have been corrupted and installing a new copy from the Play Store will ensure you’re using the latest version. 
   
   1. Swipe up from the bottom of the screen to pull up the app drawer. 
   2. Go to the screen where the Messenger app is located. 
   3. Tap and hold on its icon until options show. 
   4. Tap Uninstall and confirm it. 
   5. Once the app is successfully uninstalled, launch the Play Store. 
   6. Search for ‘Messenger ’ and tap the Messenger  app. 
   7. Tap Install and wait until the phone has finished installing the app. 
   
   When it comes to problems like this, as long as the camera is working properly on its own, reinstalling the app in question can always fix it.