a.rb

To fix grub rescue go with following steps:

    First thing is we have to start our OS only then after we can fix grub.

    #to start OS-->
    error: unknown filesystem.
    Entering rescue mode...
    grub rescue>

    When u see such error first we have to check for “Filesystem” is ext2'

    grub rescue> ls        # type 'ls' and hit enter to see drive partition.
    (hd0) (hd0,msdos6) (hd0,msdos5) (hd0,msdos4) ...   # you will see such things 

this are our drives now we have to check which one is ext2.

grub rescue>ls (hd0,msdos6)
error: disk 'hd,msdos6' not found.

go for another drives until you get “Filesystem is ext2”.

grub rescue>ls (hd0,msdos5)
error: disk 'hd,msdos5' not found.
grub rescue>ls hd0,msdos2)
(hd0,msdos2): Filesystem is ext2        # this is what we want

now set the path

grub rescue>set boot=(hd0,msdos2)
grub rescue>set prefix=(hd0,msdos6)/boot/grub
grub rescue>insmod normal
grub rescue>normal

    Now just fix grub by following command on any ubuntu

    sudo grub-install /dev/sda
    sudo apt-get update
    # to update grub
    sudo apt-get upgrade

make sure you must update grub after login into OS