Yep. First, boot from your Ubuntu LiveCD, and choose "Try Ubuntu"; let it continue until you get the default Live desktop. From the Dash, open a terminal and enter the command:

sudo fdisk -l

This is to find out the partition to which you have installed Ubuntu. It'll be sdXY, where X is the drive: it'll be a letter such as a or b; and Y is the number that has been given to the partition on that drive: it'll be a number. Find your Ubuntu partition there, if you can. You need to know X and Y. (If you can't tell which is the right partition, you can open GParted from the Dash to get a graphical view of your partitions, and maybe figure it out from there.)

Once you know that, then enter the command:

sudo mount /dev/sdXY /mnt

where X and Y are the partition that you found in the previous step. For example, if your Ubuntu partition is sda1, you'll enter "sudo mount /dev/sda1 /mnt". This is to mount the partition so that you can work with files on it.

Now you need to reinstall grub to that partition. If you're running Ubuntu 11.04 or later, enter this command:

sudo grub-install --boot-directory=/mnt/boot /dev/sdX

where X is the partition (a, b, c, etc.) that you obtained above: note that it does not take a number this time.

If your version of Ubuntu is older than 11.04, then enter this command instead:

sudo grub-install --root-directory=/mnt /dev/sdX

This will reinstall the Ubuntu grub you had been using before. When you reboot, you'll have your old grub. Once in to your Ubuntu, open a terminal and run this command:

sudo update-grub