I suspect the option you selected was the Compatibility Support Module (CSM), which is a fancy way of saying you enabled BIOS (aka legacy) support. Note that most modern computers don't have BIOSes, no matter what the manufacturers say; they have EFIs, which are completely different. Referring to them as BIOSes just causes confusion, IMHO. They can boot BIOS-mode OSes through the CSM, which is to EFI much like what
dosemu or WINE is to Linux.
If your firmware interpreted the activation of CSM as an "only-use-CSM" sort of thing, that would explain why you'd be unable to boot once you activated the CSM -- you probably had no BIOS-mode boot loader installed, so it would be like telling Linux to launch WINE and no other native Linux programs when you have no Windows programs installed -- useless.
Once you de-activated the CSM, the computer should have begun booting again; but it's conceivable yours has a bug that causes it to forget its EFI boot entries after you've activated the CSM; or maybe the CSM remained active, with its boot entry at the top of the EFI's list. I can suggest four possible solutions at this point:
- Enter your firmware setup utility and look for a boot-order option. If you see an entry called "Ubuntu," move it to the top of the boot list.
- Enter your firmware's built-in boot manager (typically by pressing some function key other than the one you use to enter the setup utility). If there's an "Ubuntu" entry, use it. If Ubuntu boots, you can then use
efibootmgr to reset Ubuntu to the top of the boot list. You would first type
sudo efibootmgr to get a list of boot options. Note the number associated with the Ubuntu entry -- for instance, it might be
Boot0005. You'd then type
sudo efibootmgr -o 5 to make "Ubuntu" (actually GRUB) the default boot loader. (You can specify a set of boot loaders to be tried in order, as in
sudo efibootmgr -o 5,1,2 to use 5, then 1 if that fails, then 2 if both 5 and 1 fail.)
- Download the USB flash drive or CD-R version of my rEFInd boot manager and create a suitable boot medium from it. You can then boot to it (using a function key to access your computer's built-in boot manager). If you can boot Linux in this way, you can then either use
efibootmgr (as above) to reset GRUB as the default or install the Debian-package version of rEFInd (directly or via a PPA; see the rEFInd page for details) to make rEFInd your default boot manager.
- Run the Boot Repair tool. This is an easy solution on the surface, but this tool often does more than it needs to do, which can occasionally cause follow-on problems.