I had set Ubuntu Hardy Heron up on one of my newer computers and then tried to boot it up on an old computer (originally running Dapper Drake without any problems). Apparently, the hardware of the old computer didn't like Hardy Heron very much and corrupted hda1, rendering the entire operating system unbootable.
After re-attaching the hard drive to the newer computer, I booted up a LiveCD and recovered the partition by mounting the partition and executing some commands in a shell.
To isolate the problem, I did some tests on the older computer, including removing the soundcard and replacing the graphics card with another one. I even tried to boot up a LiveCD, but everything just froze. Ultimately, I concluded that the problem lay with the motheboard, a Micro-Star MS-6575 (Version 3.1). If you have a computer with that, don't install Ubuntu Hardy Heron on it - at least not for now. :-)