I broke my boot loader last weekend trying to enable hibernation. It worked, but was flaky, so I decided to undo everything, but when I tried to run upgrade-grub to apply the changes, it stalled. No matter what I tried, I couldn’t make it to run, and booting without upgrading grub was out of the question.

Fortunately, since the only change was related to hibernation, shutting down instead of hibernating let me reboot using the old boot loader, and after that the update-grub worked well.

Having broken the boot loader several times before taught me that you can sometimes boot a broken boot loader in the right conditions.