Do you have more than one Drive (You may have said but I am tracking too many threads to remember off hand...)?
Do you have more than one EFI partition?
If the above questions both yield "no" as the answer,
Please try using the built Grub repair utility:
Boot into the LiveUSB Zorin OS Medium.
Ensure you have network connection.
Open a terminal and run
sudo apt-add-repository ppa:yannubuntu/boot-repair
sudo apt update && sudo apt install -y boot-repair
boot-repair
IF the boot repair utility is unsuccessful, it will create a Log file. Please post the results of that log file in this thread.