Camera is not working in ASUS Zenfone2
Backup is important


flash Stock ROM again.
Root the device, install a custom Recovery(TWRP), and install a custom ROM(Eg: Cyanogenmod or mokee or aicp etc) URL to XDA is descibed below :


XDA Zenfone2

How to Root and Install TWRP is described on this link.

How to flash:

Check for any firmware file required, before flashing ROM. Otherwise, your device will get bricked.

a. Install latest TWRP recovery on your device.
b. Make a backup of your current ROM.
c. Wipe system, data, cache and dalvik.
d. Flash ROM and Google apps.
e. Reboot system.
Custom ROM is installed.