A guy bought me his Windows 10 tablet. His camera app stopped working. The fix was simple enough. This might work with other non-working apps as well.
Click on the start button and the the Settings symbol.
Then in the Windows Settings window click on the Apps section.
In the window that opens, down the left hand menu click on Apps & features. Then scroll down to find the Camera app on the right hand side and click on it. Then click on Advanced options.
Then finally, click on the Reset button, wait till it finishes and then reboot or restart the system. This is what worked in my case
