Due to Anycubic Photon Zero problems with unresponsive Z Carriage that a lot of user have, Anycubic send us, via their after sales department, the stock firmware to be reinstalled. You can find it in this repository.
If, after flashing the firmware, it doesn't work properly, you must check motor; open the bottom cover of the machine and check the cable connection of the motor: https://youtu.be/rBOlTqj5v90