If you have one, use your live CD or a CD like Puppy or Knoppix to examine your original hard-drive. You might also be able to do this from your installed Ubuntu, but my personal preference is to use a CD-based Linux, like Knoppix, Puppy, or the Ubuntu live CD, mount, and then examine the hard drives to see what's on them.
I could be the installed just defaulted your /etc/fstab, so one of the hard-drives is not mounting. It's hard to diagnose without seeing the contents of your fstab and an output of df.
Whatever you do, don't panic. Your partitions might still be usable and not overwritten, but you'll have to examine your /boot/grub/menu.lst on what you believe was the original boot partition to see what's what.