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
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