Restore Grub

If you have your Linux system in a second (or third...) hard disk this method will not work. Please check Super Grub
Disk's method that address this problem.
1. Boot from a Live CD, like Ubuntu Live, Knoppix, Mepis, or similar.
2. Open a Terminal. Open a root terminal (that is, type "su" in a non-Ubuntu distro, or "sudo -i" in Ubuntu). Enter root passwords
as necessary.
3. Type "grub" which makes a GRUB prompt appear.
4. Type "find /boot/grub/stage1". You'll get a response like "(hd0)" or in my case "(hd0,3)". Use whatever your computer spits out
for the following lines. Note that you should have mounted the partition which has your Linux system before typing this command.
(e.g. In Knoppix Live CD partitions are shown on the desktop but they're not mounted until you double-click on them or mount
them manually)
5. Type "root (hd0,3)".
6. Type "setup (hd0,3)". This is key. Other instructions say to use "(hd0)", and that's fine if you want to write GRUB to the MBR.
If you want to write it to your linux root partition, then you want the number after the comma, such as "(hd0,3)".
7. Type "quit".