When I booted up this AM, no OS Zorin came up. Instead, I got the error; grub rescue> what command can I use to get Zorin back? Thanks.
From that prompt, you can run ls to list your partitions to see if your Zorin OS partition is present.
One solution may be to boot into Recovery Mode and reinstall Grub, then update grub, then test rebooting.
Install grub by first identifying your drive partition.
For this command, I will use the example of sdX
sudo grub-install /dev/sdX
Once that is complete, update grub:
sudo update-grub
Reboot to test.
If You should have Zorin already on a bootable USB Stick, start it and use the ''Try Zorin'' Mode. Then start in this Live Mode the Tool ''Boot Repair'' and let it run.