I installed Z16 on the SSD with Z15 and now after restart after the install I cannot at all start my PC!!!!!
it says
GNU GRUB version 2.02
Minimal BASH-like line editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists possible device or file completions
grub >
I have removed the SSD with Z16 and still have the same message immediatly at startup!!!!! I cannot even access W10 !!!!!!!!
I picked up on this because I also run multiple drives as you do and possibly similar issue (although not sure it is related).
Not sure I am the best person to help, but think you need to find a way into the BIOS and check the boot drive. If you can't get, perhaps posting a little more info about what happens might be useful. Hopefully it will be as simple as switch it back to Win 10.
It sounds like you screwed up the GRUB boot loader by installing Z16 on the same drive as Z15. And I have heard of people on here screwing up their ability to boot into Win10 as well, trying to dual boot both OS's. You going to have to run a GRUB boot repair operation.
@Aravisian has much more experience with this then I do, and can provide you the best support.
Having said that however, I can provide you one possible avenue to try. Boot off of the USB drive you used to install Zorin OS 16, but hold the left shift key down as you are booting.
When you see the GRUB loader, choose advanced options, there should be a REPAIR option somewhere in there, and that might get you going again. Will that fix Win10 installation? More then likely not, that will require Aravisians expertise.
But the boot repair option might just fix your ability to boot into Zorin OS 16, to allow you to use your computer again.
just to be clear...no I did not screwed the grub...zorin did this...and the worst (for the good) is that finally after 3-4 restart and the USB key with the iso inserted the PC could start with no issue...I have no idea what exactly solved this since I didn't made anything else than restarting the PC and inserting the USB key...