Unable to boot: Missing Operating system

Solution 1:

Your Grub file seems ok. I suggest installing Grub2 manually again. First, start Ubuntu from USB stick and do not mount your installed Ubuntu. Open a terminal and apply those commands:

sudo mount /dev/sda1 /mnt
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
sudo chroot /mnt
grub-install --force /dev/sda

You can check if it is OK or not with:

grub-install --recheck --force /dev/sda

If everything is ok, unmount all the things and quit:

exit
sudo umount /mnt/dev/pts
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/sys
sudo umount /mnt
sudo reboot