As Ocka has said above, you somehow need to get to your Grub menu and from there select "Advanced Options for Zorin". That should display previous linux kernel versions prior to your update. Try one of those to see if it was a kernel update that has failed you.
See: [HOW TO] enter Recovery Grub Menu (Advanced Options for Zorin)
PS:
You need to tap ESC like a manic woodpecker at startup, not hit it once or hold it down.