I've just installed Zorin OS to my daily driver laptop, and GRUB doesn't want to show up. It only showed up once, but it still works. I could choose between Zorin and Win 10, so it must be some graphical error. Do you have any ideas?
You could try Boot Repair:
I just dealt with this same issue, and learned a few things:
- If your computer uses UEFI rather than BIOS (which it probably does), the key to bring up GRUB is ESCAPE, not SHIFT.
- It is possible to hit this button too early.
- (Related, but I'm less sure about this one:) Some instructions say to hold down the button, but I found that this didn't word. I had to repeatedly mash it (but not too early)