Warning
Use a sudo-capable account, keep shell history for audit, and back up critical paths before changing boot, SSH, or package state.
What you will achieve
Get Linux booting again when updates or Windows overwrote the bootloader.
1) Boot from Ubuntu live USB and mount system
sudo fdisk -l
sudo mount /dev/sdXY /mnt2) Chroot and reinstall GRUB
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt
grub-install /dev/sdX
update-grub3) Reboot and verify boot order
Check UEFI boot order with efibootmgr -v if needed.