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