Check vcgencmd get_camera result.
It will return how many cameras are supported and detected.
Normally you will get "supported=1 detected=1".
If you get "detected=0" then camera is not detected : double check your connection, ribbon cable integrity and orientation, or consider that a picamera can fry from times to times.
I never met "supported=0", but ...
...in /boot/config.txt solved the problem for me with a Pi Zero, good quality power supply and low quality 3rd party knock-off cheap camera.
My guess is that 3rd party cheapo cameras draw more volts that the board expects, and that over volting the board compensates for this.