Comment 1 for bug 739637

Sometimes the drive orders get mixed up e.g. if another device is plugged in - or for reasons unknown. So for the ubuntu install the hard drive is recognised as /dev/sdb but on the next reboot it's /dev/sda. This is a possibility. So to rule out, hit 'e' when you see the grub menu, change /dev/sdb2 to /dev/sda2 and hit CTRL+x to boot. You can also check by hitting 'c', and entering ls (lowercase LS) to see the partitions listed. If you see (hd0) (hd0,1) (hd0,2) (and there's no (hd1,2) == /dev/sdb2) then you can be sure it should be /dev/sda2
Once booted go to a terminal and update grub (CTRL+ALT+t, sudo update-grub)

That's a possibility but there could be another cause. But try it first.