You can reinstall Grub2 using your Ubuntu live stick.
Mount the partition your Ubuntu Installation is on. If you are not sure which it is, launch GParted (included in the Live CD) and find out. It is usually a EXT4 Partition. Replace sda1 with the drive and partition number your installation is on, for example:
sudo mount -t ext4 /dev/sda1 /mnt
Now bind the directories that grub needs access to to detect other operating systems, like so.
sudo mount --bind /dev /mnt/dev &&
sudo mount --bind /dev/pts /mnt/dev/pts &&
sudo mount --bind /proc /mnt/proc &&
sudo mount --bind /sys /mnt/sys
Now we need to mount chroot:
sudo chroot /mnt
Now install, check, and update grub.
This time you only need to add the drive letter (usually a) to replace , for example:
grub-install /dev/sda
grub-install –recheck /dev/sda
update-grub
Now grub is back, all that is left is to exit the chrooted system and unmount everything.
exit &&
sudo umount /mnt/sys &&
sudo umount /mnt/proc &&
sudo umount /mnt/dev/pts &&
sudo umount /mnt/dev &&
sudo umount /mnt
Shut down, take out the usb-stick and turn your computer back on, and you will be met with the default Grub2 screen.
If you want to put grub on your internal SSD ( where your Windows is installed ) you need to set it to your internal drive.
Although I don't see the point in this ( without the external disk it cannot boot into Ubuntu. ( see gparted ).
In my case nvme0n1. see picture
If you want to put grub on your external SSD use that drivename. In my case
sda ( external usb-stick ). see picture
If you put grub2 on your external disk and want to boot from it you have to change your settings in your ( what many people still call ) BIOS to boot from usb first.
And Nixie has a video on youtube about this: enter link description here
This should get you booting into Ubuntu again.
