You could boot from live Zorin USB boot stick, choose "Try Zorin" mode and then enter "boot repair" in the search field of the Zorin start menu and run it.
Another option:
Follow the first steps as described in this thread.
First enable os-prober (it is a service that detects other OS on the installed system) by editing /etc/default/grub/ so that this line is shown there
GRUB_DISABLE_OS_PROBER=false
then run
sudo os-prober
and if the output shows Windows update grub with
sudo update-grub
followed by a reboot.