Hi @ricmarques. I was a few steps ahead of you and realized GRUB was present, just hidden. So, I went and changed the /etc/default/grub file and ran the sudo update-grub command. But this made it only worse. Now, the boot process winds up with a black screen with a white cursor on the upper left corner. No keyboard input, mouse, etc. are recognized.
I have now spent the better part of last night and today attempting a variety of solutions. I have burned to bootable USB drives Boot Repair Disk, Super Grub Disk, Rescatux and a full .iso of Ubuntu-MATE 20.04.5 LTS. I have been through the UEFI boot process so many times I can pick out the appropriate F key when needed, in the dark!
Every one of these "rescue" disks at least loads. Super Grub at least let me boot into Linux, and even after re-installing grub, failed to give me a clean boot. At least fsck didn't present me with any disk errorrs. I confess, I'm baffled.
I have been trying to avoid re-installing Ubuntu-MATE, but it looks like that may be my last resort. I do not have 12T of backup storage, so I'll likely be looking at preserving that which is most important (and least re-creatable). I'll probably slice the large disk into partitions and copy what I can over.
What's most frustrating about this is that when Windows or Mac collapses, everything becomes a dust heap. Here, I have Linux, and can boot into it, but there isn't much I can do with it at this level.