Grub bootloader repair

If grub does not load and freezes when you are on the opening screen and you remain stuck on the command line, you can access "grub" again by executing the following commands one by one.

group rescue> ls
grub rescue> set boot=(hd0,gpt*) #may change.
grub rescue> set prefix=(hd0,gpt*)/boot/grub
group rescue> insmod normal
group rescue > normal

sudo update-grub

sudo grub-install /dev/sda