Changing /etc/X11 would not affect grub or linux, just the Desktop showing up.
- To get to the BIOS you would select the ‘F2’ key for most machines, and the 2020 version of the tensorbook should be the ‘F2’ key.
Then you could be from a USB device also if you need to recover, once you are in the BIOS or first you could see if the correct drive is selected.
It sounds like a kernel or boot parameter may have been changed or not completed.
But to be sure you can boot from a USB ‘install’ CD and select ‘Try Ubuntu’ and mount/look at the other installed drive.