Sometimes, in case of troubleshooting, you may have to install a new GRUB bootloader.
So normally GRUB2 is installed automatically, and you don't have to worry about it. The
only thing you would do is to use GRUB2 mkconfig to write new settings to the GRUB bootloader
configuration files. To write a GRUB2 bootloader to a completely new device, you can use the
GRUB2 install command. And installing or reinstalling GRUB2 can be useful for troubleshooting. On
BIOS systems, use GRUB2-install-dev-sda, and that will be enough to reinstall GRUB2. Obviously,
you do need to verify your storage device. It should be your primary disk, and on many
systems that should be dev-nvme-0-n1 instead of sda. If you are on UEFI, it's a little
bit more complex, because in UEFI, there is also the cryptographic keys that are required
for working with Secure Boot. And they need to be reinstalled as well if you are going
to reinstall GRUB. To do so, you would use the command GRUB2-reinstall-grub2efishim,
where the shim file contains the cryptographic keys that allow for working with Secure Boot.
And if you have done all that, while you reinstalled GRUB, and on a system that had
become unbootable, it should be bootable again.