Most prefer Boot-Repair as if it is just repair the MBR, all you have to do is click on the automatic repair. All the other options are for more complicated issues.
I may have posted before a dd way of copying the MBR, but find dd so dangerous that I will not suggest that. It is just too easy to repair. If you do not want to use Boot-Repair you can type two lines from a current version Ubuntu liveCD if you know the partition you have grub installed into. If you have a separate /boot you have to also mount that first. Example below uses sda5, change to your partition.
#Comments are anything after the #, enter commands in terminal session
#Install MBR from LiveCD/usb, Ubuntu install on sda5 and want grub2's bootloader in drive sda's MBR:
#Find linux partition, change sda5 if not correct:
sudo fdisk -l
#confirm that linux is sda5
sudo mount /dev/sda5 /mnt
sudo grub-install --root-directory=/mnt/ /dev/sda
# The above command should work but they now suggest this command for grub 1.99 with Natty 11.04 or later - uses boot not root.:
sudo grub-install --boot-directory=/mnt/boot /dev/sda
# If no errors on previous commands reboot into working system and run this:
#How to restore the Ubuntu/XP/Vista/7 bootloader (Updated for Ubuntu 9.10)
#Reinstall grub2 - Short version