I have had my laptop set up to dual boot into Windows 10 or Zorin 17 core for several months. Love Zorin.
Today I used the Zorin Upgrade tool to go from 17 core to 18 core but after it requested a reboot to complete the upgrade, I find that my laptop now goes straight into Windows 10!
The Grub menu was set up when I installed Zorin 17 core alongside Windows 10 on my laptop. The Grub menu has been working for months until yesterday when I did the upgrade to 18 core. I modified the Grub to change the default from Zorin to Windows 10.
Do You still have in BIOS and ''ubuntu'' Entry? Using Boot Repair like @Aravisian already suggested is worth a Try, too. When You still have the Zorin Bootstick, start it and run it in Live Mode. there is Boot Repair available.
And in Live Mode GParted is available, too. Open it and make a Screenshot/Picture from the Partitions of the Drive and post it here.
GRUB_TIMEOUT_STYLE=hidden is what it was in current state will show you the menu everytime you boot up is all for 10 seconds Glad you got it worked out
PaulMcl, interesting question of which makes perfect sense to me as to why does Zorin change the /etc/default/grub file when upgrading from 17 to 18? If it aint broke don't fix it. Unfortunately, the upgrade broke my Zorin/Windows 10 dual boot setup.