This may relate to changes you mentioned having made but do not fully remember, earlier.
You might try using the grub parameter:
But I am beginning to wonder if you have another grub file set that is affecting your boot. This may make sense as you had a blank file before... and you need a functional one.
We would need to locate that file. And since you are not certain about your memory of what you did before, that could be difficult to perform remotely.
One option would be for you to Reinstall Zorin OS.