Grub-rescue notes:

We are assuming that the linux installation has suffered some damage where the grub installation has lost the location of the grub config files and therefore can't boot in the normal way. This will often result in the system failing to boot and the prompt

    grub-rescue>

is displayed.  The following notes outline a relatively straightforward way to point ot the installed grub files and hence sucessfully boot.

First we want to see if we can find any linux partitions - we use the ls command to do this.

for example ...

 grub-rescue> ls
 (hd0)  (hd0,msdos2) (hd0,msdos1)    -  disk is hd0 with a legacy msdos style
                                        partition table with 2 partitions.

 grub-rescue> ls
 (hd0)  (hd0,gpt2) (hd0,gpt1)        -  disk is hd0 with a GPT partition table
                                        with 2 partitions.


Next, we want to see if there are any linux files on the partition.
 
grub-rescue> ls (hd0,1)/
lost+found/ bin/ boot/ cdrom/ dev/ etc/ home/  lib/
lib64/ media/ mnt/ opt/ proc/ root/ run/ sbin/ 
srv/ sys/ tmp/ usr/ var/ vmlinuz vmlinuz.old 
initrd.img initrd.img.old

Now see if we can find a set of grub files ...

grub-rescue> ls (hd0,1)/boot/grub
fonts  gfxblacklist.txt  grub.cfg  grubenv  i386-pc  locale  unicode.pf2

We can now set grub to use the  grub files we have discovered on the partition to attempt to boot.

grub-rescue> set prefix=(hd0,1)/boot/grub
grub-rescue> insmod normal
    
    this will set up for booting into (hd0)
    
then start the boot sequence with

    grub-rescue> normal
    
With a bit of luck, you should get a clean boot.  After logging in, you will need to run, as root, the grub install and update commands.
    
For example - for ubuntu and derivatives
    
    $  sudo grub-install /dev/sda
    
    $  sudo update-grub

Now confirm that you can shutdown and reboot in the normal way.