Windows 7 replaced GRUB2

How to reinstall GRUB2 when Windows replaces / removes it  

Provided your Ubuntu installation is working correctly...

Ideally run the Ubuntu LiveCD for your version of Ubuntu

Mount the drive with your existing GRUB2 directory to somewhere like: /mnt/tmp

Now run:
grub-install --boot-directory=/mnt/tmp/boot /dev/sdX

NOTE: do not put a partition number, just the device (/dev/sda for example)