I use two operating system in a single pc. One is WINDOWS and another one is UBUNTU (Linux). I was using windows and partitioned a drive and restart. Suddenly my GRUB ( boot menu ) just vanished and showing the message “ unknown filesystem, grub rescue ”. My fellow colleague Tanzim helped me a lot to resolve the situation to recovery the system. We reinstalled the GRUB and it worked fine. Every file and folder, filesystem were intact. What I lost, the windows selection menu in boot menu. We searched the INTERNET and found a way to load the windows selection menu again in the boot menu. Let me show the way to do it.
First Reinstall the GRUB.
For this you need a UBUNTU live cd or UBUNTU image on a pendrive.
a. insert the cd or pendirive, restart the pc and select the boot device as pedrive/cd
For Ubuntu grub rescue follow these steps:
1.Boot from the Ubuntu Desktop live CD.

2.In Live Desktop session open terminal.
Application ->Accessories->Terminal.
3. In Terminal tpye sudo fdisk -l
It will display all partiton of the disk.

The partation which have Linux under System column is your drive in which ubuntu linux is installed. In screenshot ubuntu partition drive is /dev/sda11.
4. Mount the ubuntu partition drive
sudo mount /dev/sdXX /mnt(example ‘sudo mount /dev/sda11 /mnt’ ,don’t miss the spaces.)
5.Only if you have a separate boot partition:
sudo mount /dev/sdYY /mnt/boot.
6. Mount the virtual filesystems:
sudo mount –bind /dev /mnt/dev
sudo mount –bind /proc /mnt/proc
sudo mount –bind /sys /mnt/sys
7. To ensure that only the grub utilities from the LiveCD get executed, mount /usr
sudo mount –bind /usr/ /mnt/usr
sudo chroot /mnt
8. If there is no /boot/grub/grub.cfg or it’s not correct, create one using
update-grub
or update-grub2
9.Now reinstall Grub
grub-install /dev/sdX(eg. grub-install/dev/sdaDo not specify the partition number.
10. Verify the install
sudo grub-install –recheck /dev/sdX
11. Exit chroot : CTRL-D on keyboard.
12. Unmount virtual filesystems:
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/sys
If you mounted a separate /boot partition:
sudo umount /mnt/boot
13. Unmount the LiveCD’s /usr directory:
sudo umount /mnt/usr
14. Unmount last device:
sudo umount /mnt
15. Reboot.
sudo reboot.
This should reinstall your grab, but you might not see the windows menu right now
what your need to do is as following
from your terminal
1. sudo apt-get install os-prober
2. sudo update-grub
3. reboot
sudo reboot
Hope it will restore your windows name in boot menu.