Restore GRUB quite simple in Ubuntu, instead going through all the "gain root access" and play with shell commands, you can use the Ubuntu installation CD to restore it without going through all kinds of hassles.
Here are the steps:
1. Boot your computer up with Ubunto CD
2. Go through all the process until you reech "[!!!] Disk Partition"
3. Select Manual Partition
4. Mount your appropriate linux partions
5. DO NOT FORMAT THEM.
6. Finish the manual partition
7. Say "Yes" when it asks you to save the changes
8. It will give you errors saying that "the system couldn't install ....." after that
9. Ignore them, keep select "continue" until you get back to the Ubuntu installation menu
10. Jump to "Install Grub ...."
11. Once it is finished, just restart your computer