grub rescue> ls [Enter]

Then try first option / if not works and make error use next one ..

grub rescue> ls (hd0,msdos7) [Enter]
(hd0,msdos7) : Filesystem is unknown .

grub rescue> ls (hd0,msdos6) [Enter]
(hd0,msdos6): Filesystem is ext2.

grub rescue> set root=(hd0,msdos6) [Enter]
grub rescue> set prefix=(hd0,msdos6)/boot/grub [Enter]
grub rescue> insmod normal [Enter]
grub rescue> normal

Then Select Ubuntu and start

Open Terminal

$ sudo update-gurb [Enter]
Password
once it load finish & Done

$ sudo grub-install /dev/sda [Enter]