Upgrade to 20.04 from 18.04 GRUB menu hidden
Same problem though I installed Ubuntu 20.04 instead of upgrading.
A way for that:
edit /etc/default/grub
to change GRUB_TIMEOUT_STYLE=hidden
to GRUB_TIMEOUT_STYLE=menu
and remove #
in front of #GRUB_TERMINAL=console
. Finally, run sudo update-grub
in terminal