Maybe it is neccessary to activate Your GRUB Menu. To do that, start in Zorin. Open the Terminal and type
sudo nano /etc/default/grub
to work on the GRUB Configuration. You will see there a white Textblock:
The Lines 2 and 3 on the Picture are important here. To have an active GRUB Menu they have to be like this:
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=20
The last one makes that the GRUB Menu appears for 20 Seconds before it start in the default Option. When this is not enough Time for You, you can change the 20 to a higher Value if You wish.
When You have make the Changes press ctrl+o to save, Enter to confirm and ctrl+x to exit. Back in the normal Terminal Overview type sudo update-grub to make the Changes affect. Don't forget that! Without it, it will not work.
After all this reboot and You should see the GRUB Menu. Selest there ''Advanced Options'' and use the older Kernel to start.