GRUB MENU
Command: ls
It will list all your drives and partitions, like:
(hd0) (hd0,gpt3) (hd0,gpt2)
If you don't know your Ubuntu boot partition, check them one by one:
ls (hd0,gpt22)/
ls (hd0,gpt3)/
When you hit the right one, you'll get a line mentioning lost+found, more importantly boot, and so on.
Assuming (hd0,msdos2) is the right partition:
set prefix=(hd0,gpt32)/boot/grub
set root=(hd0,gpt3)
insmod normal
normal
Now you'll be able to boot into Ubuntu. Once you do, execute the following commands:
sudo update-grub
sudo grub-install /dev/sda