If your GRUB menu is not displaying and/or the shortcut key is being ignored, you can force a timed display as follows:

Edit the GRUB configuration:

sudo vi /etc/default/grub

If you see a line like GRUB_HIDDEN_TIMEOUT=0 or GRUB_TIMEOUT_STYLE=hidden, comment it out:

#GRUB_HIDDEN_TIMEOUT=0

Then, specify the amount of time you want the GRUB menu to display on startup, where ‘#’ is seconds:

GRUB_TIMEOUT=#

After making changes, run sudo update-grub to update the grub configuration.