before I go into some list of try this, try that stuff. Lets start with a simple question to you. Was your phone working before you flashed that stock backup?
If you can boot to boot menu, (hold volume up while turning on)
First try to boot recovery and do a reset.
If that does not help do these things:
Then -open to fastboot
- flash twrp
- boot to menu again
- open recovery
- flash magisk
If cannot get to boot menu.
- use sp flash tool
- flash patched boot.img