Why won't Samsung Galaxy S4 mini camera work when making video calls?

I have a problem with my camera. I have viber on my phone but when i want to do video calls it doesn't work for myself the picture And when i hang up and go to use my camera it says camera failed what should i do? 




It is the android firmware update 4.4.2 downgrade


I have video call my sister on her Samsung s4 mini and it works but her front camera does not work.we can both see me but not her.is there any way to fix this problem?
Help will be appreciated.



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