Here some instructions on how to show or display the grub2 menu during the Ubuntu startup.
from the menu select Applications - Accessories - Terminal and write the command:
sudo gedit /etc/default/grub
search for this line:
well, this way the menu is HIDDEN
if we want to SHOW it at every startup, we need to comment out that line using # ,like this :
save the opened file after the change
then write this command:
as soon as it finish to update everything, we can reboot the system and see the new menu behaviour.
To HIDE it again simply do the same, remove the # symbol, save, run update-grub and it's done