You may need to reinstall grub when windows is installed after ubuntu (or if you reinstalled windows in dual boot (Windows and ubuntu) system). I used following steps for reinstalling grub in order to login to my Ubuntu 10.04 after reinstalling Windows 7 on my machine.
1. boot the ubuntu LiveCD
2. Open a terminal and type
sudo fdisk -l
This list the partition tables, and in my case root (/) partition is on /dev/sda9.
3. Mount that partition using following command.
sudo mount /dev/sda9 /mnt
4. Run the grub-install command as below. This will reinstall the grub 2
sudo grub-install --root-directory=/mnt /dev/sdaHere "sda" is the hard disk on which your Linux distribution is installed!
6. Finally Refresh the grub 2 menu using following command
Now everything should be fine :)