Repair GRUB from a live USB

Get booting back after a Windows update or a mistake.

1. Boot from a live USB of the same architecture and open a terminal.
2. Mount the root (adapt sdXn) then the EFI partition:

   sudo mount /dev/sdXn /mnt
   sudo mount /dev/sdY1 /mnt/boot/efi

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3. Mount the virtual file systems and enter the system:

   for d in dev dev/pts proc sys run; do sudo mount --bind /$d /mnt/$d; done
   sudo chroot /mnt

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4. Reinstall and regenerate GRUB (Debian/Ubuntu, UEFI):

   grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB
   update-grub

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5. Leave with exit, unmount, reboot.