I have owned a couple of Xiaomi phones in the last few years, and because they are not the most affordable smartphones you can buy, I have recommended that to everyone in my family and extended family.
Which makes me the unofficial tech support guy for Xiaomi phones, at least for my extended family. I try to solve almost all of their problems that are related to software.
One of the very common problems that have come my way is the camera error. Especially, the camera error in Redmi Note 4. Maybe because it is the most sold phone in the budget segment in India.
If you two are facing a similar issue with your Redmi phone, here are the possible solutions for the issue
Try restarting your phone
I know you are aware of it but this is the first step towards troubleshooting any error in your smartphone, I can guarantee that 90% of the software bugs can be solved just by restarting the smartphone.
Check the camera permissions
If this particular issue you are facing is inside a particular app, such as the WhatsApp camera or the Instagram camera, then it could be a Permissions issue. To use the hardware of your smartphone required permission to access the particular hardware, switch off the camera, mic, etc.
There are two ways to give the permissions back to this particular app, here is the easier one.
You need to go to the settings page of the particular app and clearing the cache and data then launching the app again, it will ask for permission to access the hardware of your smartphone, which you can allow.
Solved this by doing the following:
1. Go to Settings -> About phone
2. Tap 5-6 times on “Kernel version”. This will enter to Engineering Mode (CIT)
3. From “QC test” menu, go to option “12.FrontCamera“
4. Camera app will start with the front camera. Now change camera from front to back. You should have image from sensor.
5. Tap back to exit the camera app, and select “Fail”. This will reset the sensor and sensor lens.
6. Exit CIT mode and test your camera. If was a problem with the sensor (or lens), it should be solved now.
I hope that either of these methods would work for you and you are able to solve your Camera issue.
In case you are not able to do that, chances are, it might be a hardware related issue, and you need to visit the mobile phone repair shop. Your first priority should be to visit an authorized Service Cente for Xiaomi Smartphones.