Solved this on a machine this afternoon. It seems that one cause of this problem is the installer thinking that you have EFI secure boot, when you don't and therefore loading the incorrect GRUB files.
What you need to do is install GRUB 2. To do this you need to boot to the live instance, mount your root partition and install.
From a live instance, find the partition on which your root partition is loaded. GParted will tell you this, or you could use
sudo fdisk -l
Go for the partition in which ubuntu is installed.
Once you have your partition you need to mount it. Assuming the root partition is on /dev/sda5, that'd be:
sudo mount /dev/sda5 /mnt
Then install GRUB 2
sudo grub-install /dev/sda --root-directory=/mnt
[use copy and paste for this one as there are some spaces that you need to get right.]
Assuming this is your problem, then you should just be able to reboot and everything will work fine.
Original solution for this was from here: http://ubuntujournal.blogspot.com/2012/11/fix-new-install-of-ubuntu-1210-wont-boot.html