I'm having exactly the same issue.
I upgraded my box (Athlon, 378MB RAM, 40GB IDE HD on ext3 - I've been upgrading
every six months since version 5.04).
As it was installing the upgrade the error messages started (did not take note
and can´t access anymore)
After rebooting, same symptoms as you mention.
Even worse: when starting with the LiveCD, the HD can be seen, but not mounted.
I diagnosed the HD with LiveCD and it did not find anything wrong
I left it there for further analysis - did not have time to do much more yet.

Maybe it is not a HD crash, as I suspected... that would be good news.