I have come across this situation on my Samsung Note 4 several times and figured out how to solve it. Bear in mind it happens after every Viber update which MEANS VIBER HASN'T FIGURED OUT THE SOLUTION YET OR DON'T KNOW THERE IS A SHITTY BUG GOING AROUND IN THEIR SOFTWARE
So here's the solution. It might do the trick or it might not work for you. Depending on what version of Android and hardware you're using (Iphone, Samsung, HTC)
1. OPEN YOUR VIBER SETTING PAGE
2. GO TO PRIVACY SETTINGS. DISABLE/UNCHECK THE COLLECT ANALYTICS BUTTON
3. CHECK YOUR VIDEO SETTINGS. IT SHOULD BE ENABLED
Now wasn't that easy?