Boot a Linux from e.g. an USB stick. It has to be a matching version to the system you attempt to repair.

mkdir destsys
mount --bind /proc destsys/proc
mount --bind /sys destsys/sys
mount --bind /dev destsys/dev

# for efi find the efi-partition
mkdir efi
mount /dev/efi-partition efi
mount --bind efi destsys/boot/efi
# end efi

chroot destsys
grub-install boot-device

# for multi boot edit /etc/default/grub and add or set
GRUB_DISABLE_OS_PROBER=false
# 

update-grub
reboot

• You need to be ROOT to accomplish this.
• Tested on Ubuntu 22.04 March 2026