Well, I wonder if adding nomodeset to grub is all you need. You may need a different Nvidia driver. But you must access the build in order to do either.
If you use the LiveUSB, you can mount the partition to update grub: https://itectec.com/ubuntu/ubuntu-how-to-run-update-grub-from-a-livecd/
Following the above guide, before running sudo update-grub, you will need to run sudo nano /etc/defaul/grub
then add nomodeset behind "quiet splash" to make it "quiet splash nomodeset"
Then update-grub.
Unless anyone else has any other suggestions... Since you are able to use the LiveUSB, I would run that to access your personal files and back them up if you have not already...
And suggest going ahead and reinstalling Zorin OS.