Edit /etc/default/grub and change the line…
GRUB_TIMEOUT_STYLE=hidden
… into …
GRUB_TIMEOUT_STYLE=menu
Then, open up a terminal and run… 
sudo update-grub
Edit /etc/default/grub and change the line…
GRUB_TIMEOUT_STYLE=hidden
… into …
GRUB_TIMEOUT_STYLE=menu
Then, open up a terminal and run… 
sudo update-grub