Asus Zenfone 3 Zoom broken power/volume key - How to boot
plug in your phone.
immediately after plugging in , hold the volume down key until asus splash screen appears.
now connect your phone to pc.
install adb ("sudo apt install adb" is the command in linux).
enter "adb devices" command.check the device is appearing on your terminal.
then give "adb reboot" command.

This worked for my asus phone.Thank you