Repairing the Grub Bootloader

If grub does not load on the opening screen and remains on the command line, you can access "grub" again by executing the following commands

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