Recovering lost Ubuntu installation after installing windows

You can  try installing and using Boot-repair, for instructions on installing and using it refer to the following link. (Note: i have not personally tried it but seems user friendly)
https://help.ubuntu.com/community/Boot-Repair

Or alternately you can choose to do it manually

Boot from a live cd of Ubuntu 10.04.
First, find out about your drives.

sudo fdisk -l

Then check to see if the drive is already mounted or not

df

If it is not mounted then mount it first. This is the drive in which Ubuntu is installed. In this example i will be mounting /dev/sda2 on to the folder /home/mtf

mkdir /home/mtf
sudo mount /dev/sda2 /home/mtf

In case you are not sure which drive has Ubuntu installed, you would like to look for a drive with System field as "Linux" in the output of sudo fdisk -l
If you have multiple such drives and are confused, then you can check the drives by mounting each of them and looking at the contents to see which is which. You can mount multiple drives on different folders using the steps mentioned above. You may also mount to the same folder (not consecutively) by unmounting the former drive.In this example /dev/sda2 is unmounted and /dev/sda3 is mounted in its stead

sudo umount /home/mtf
sudo mount /dev/sd3 /home/mtf

Once the drive is mounted

sudo grub-install --root-directory=/path-of-mounted-drive-with-ubuntu-install /dev/sdx

Replace x with the letter for your hard disk.

In my example, /dev/sda3 is the drive with Ubuntu installation, mounted on /home/mtf so the command would be

sudo grub-install --root-directory=/home/mtf /dev/sda

Note that in the above case it is /dev/sda and NOT /dev/sda2

Additionally to ensure everything is in order boot into your Ubuntu and then do the following to fix any missing or malfunctioning entries in grub

sudo update-grub

https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows