You can try re-installing the grub using the Ubuntu Live CD, in two different ways.
GUI
1. Boot your computer up with Ubuntu CD
2. Go through all the process until you reach "[!!!] Disk Partition"
3. Select Manual Partition
4. Mount your appropriate linux partions / /boot swap .....
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
Command line
1. Boot your computer up with Ubuntu CD
2. Open a terminal window or switch to a tty.
3. Go SuperUser (that is, type "sudo -s"). Enter root passwords as necessary.
4. Type "grub"
5. Type "find /boot/grub/stage1". You'll get a response like "(hd0,1)". Use whatever your computer spits out for the following lines.
6. Type "root (hd0,1)", or whatever your hard disk + boot partition numbers are for Ubuntu.
7. Type "setup (hd0)", to install GRUB to MBR, or "setup (hd0,1)" or whatever your hard disk + partition nr is, to install GRUB to a partition.
8. Quit grub by typing "quit".
9. Reboot and remove the bootable CD.
