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
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