How to fix "Unknown filesystem. grub rescue.." error

1.  Switch on the laptop, wait till you get the grub rescue screen
2.  Type ls command
3.  It will show a list of all partitions, something like this (hd0), (hd0,msdos1), (hd0,msdos2)
4.  You need to find the root drive for linux. So type set prefix=(hd0,msdos1)/boot/grub command
5.  If you get an error message, try the same command with other drives
6.  If no error message is shown then continue typing the following commands
7.  insmod normal
8.  normal
9.  Your PC will now boot successfully. Now login into the linux and type the following commands in terminal
10.  sudo update-grub (if it does not work then try sudo grub-update)
11.  sudo grub-install /dev/sda

This should do the job.