Grub uses unique identifier that changes after upgrading the OS or Exporting, then Importing the .OVA file on a VM Server.
Symptoms: Will not boot after upgrade.
Root Cause: Drive's unique identifier changes after OS upgrade or installing new image. This is preventable before rebooting if you change the way grub is configured to boot to using "by-label". This happens because the drives UUID changes when the drive is cloned.
Download and boot to "Boot Repair Disk" ISO image. Load the CD/DVD image with VirtualBox client and reboot. After the Boot Repair disk loads, press the "Automatically Fix Bootloader" button when prompted.