There are many possible causes to a GRUB break: Windows writing on the MBR, DRM preventing GRUB from installing correctly, installer bug, hardware change...Updating GRUB as proposed initially by Scott is generally not sufficient, reinstalling GRUB as proposed by Marco is more efficient, but still there are various situations requiring other tweaks (adding options to kernel, unhiding GRUB menu, changing GRUB options, choosing the right host architecture...).Other difficulties for repairing GRUB is the use of chroot, and the choice of the right partitions /disks.