no ubuntu menu after reinstall second os

i have 3 os in my pc. with 5 partition

1. Xp in 1st partition
2. W7 in 2nd partition
3. Ubuntu lts in seperate partition which was installed outside windows.

All was working fine,

I have reinstalled w7, after the reinstallation ,the ubuntu menu is missing. I think when i reinstall the w7 , the boot loader might have wiped out.

I inserted live cd and i follow the instruction in a webiste

1. Once you have the ununtu live CD ready, boot your computer with this CD.

2. On first menu, select the language > English.

3. Select the option called “Try Ubuntu without any change to your computer”. This will launch ubuntu in live mode.

4. Once it loads the Ubuntu desktop, go to Menu > Accessories > Terrminal. This will open a prompt window.

5. On this prompt window, type command “sudo grub” without the quotes and press enter key. This will show a grub prompt.

I tried the above instruction

it say

To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

ubuntu@ubuntu:~$ sudo grub
sudo: grub: command not found
ubuntu@ubuntu:~$

i have chek the partition the ubuntu os still there also there is a folder named boot and threre is a folder name grub

now how to boot to ubuntu os please inform me