Ubuntu – Tried to boot Ubuntu, but the GRUB rescue prompt shows up instead

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 you see such an 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

If you installed from a USB live media, you are probably experiencing bug 384633, where the initial creation of the grub boot config file uses a wrong device instead of a UUID. If you can boot off the live media, you can reinstall grub to the external usb disk, and that should fix the grub config file. You can also edit the grub boot commands (commands on screen to edit and boot) to fix the wrong devices. For the last few years, the fix was to simply reduce the disk letter or number by one, and upon successful boot, immediately run

 sudo update-grub

With 12.10, the wrong device has changed, it may even be sda (which is usualy the internal hard disk). In 12.10, I have started seeing the disk lettering change, with the internal hard disk sometimes sdb instead of sda -- not even consistent. But after the update-grub, UUIDs will be used, and the disk letter will no longer matter. Feel free to add yourself to the bug, and maybe someday it will be fixed.