How to get to Grub menu on 20.04?

I've got this error when I try to boot my desktop up.

/tmp/mountroot-fail-hooks.d//scripts/init-premount/lvm2: No such file or directory

I'm running "on the metal" or whatever, no dual-boot anything.

I recently upgraded to 20.04 LTS from 18.04 LTS, but it ran fine until I started upgrading things and ran "autoclean", then reset.

I read that I should try for the Grub menu, and tried pressing ESC at the 5 dots loading screen, but holding it down just made it flicker between some kind of commandline / terminal, and the 5 dots.

I tried pressing ESC and holding C or any of the SHIFT keys during the terminal...

None of those got me to the Grub menu, whose appearance I looked up.

Oooh... Trying the "normal" command and hitting ESC while it was running got me to the GRUB Menu! Unfortunately, even doing the safe-mode parallel and running the package restorer didn't help.

How do I get to the Grub menu on 20.04 LTS Ubuntu?

If at 5 dots it usually is too late.

You use Escape key after UEFI screen but before grub normally shows.
Or if BIOS you press an hold shift key from BIOS screen until grub menu loads.

If you have UEFI fast boot on, you may not have time to press any key.
And if you changed grub timeout to 0 you may not be able to get grub menu.

Cold boot or full power down, may then then have system go back thru the full POST and give you just enough time to press keys.

If grub timeout at zero, check Boot-Repair's advanced options or a full reinstall of grub.

https://help.ubuntu.com/community/Boot-Repair

https://sourceforge.net/p/boot-repair/home/Home/