REPAIR GRUB2

REPAIR GRUB2 AFTER WINDOW7 INSTALLATION 

1:sudo -i

2:fdisk -l

output:
Disk /dev/sda: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000080

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1           12750       19458    53882881    5  Extended
/dev/sda2              13        7650    61337600    7  HPFS/NTFS
/dev/sda3   *        7651       12749    40957717+   7  HPFS/NTFS
/dev/sda5           12750       19458    53882880   83  Linux

so linux is installed in sda5 as you can see above.

3:mount /dev/sda5 /mnt

4:mount /dev/sda5 /mnt/boot            #skip this one if not have a separate /boot partition

5:grub-install --root-directory=/mnt/ /dev/sda

6:sudo reboot

voila your dead ubuntu is up !!!