Do you see the grub menu on boot?
If you do see it, try running previous linux kernel:
From menu item: "Additional Options for Zorin"
Choose previous linux kernel generic version
boot that and see result.
What graphics do you have?
or. What PC make/model do you have (so we can find that out).
I will move this thread to General Help, as it is a post-installation issue.
Do all 5 machines affected have ZorinOS 18.1 Core installed?
EDIT: Are these new ZorinOS installations or were they all working before the software update?
FYI. Some pre-installation advice can be found here: Before you install