Hi
We could use a little more information.
Are you saying that the computer won't boot?
How do you know that the master boot record is missing?
If the computer has important data on it that can't be lost, one thing you could do is create a bootable Ubuntu DVD using another computer, so that you can boot the computer and back up the files to disks or an external hard drive.
Once the data is safe you can work on restoring the computer but you don't want to risk losing the files.
http://www.ubuntu.com/download/desktop
Once that's done you could restore the computer, either by resetting it to factory state or doing a restore.
Do you have a Windows install disk...