Ok I just could run the Ubuntu 11.4 64bit live CD on the server... and the files are still in there.
So this probably just means that my storage controller drivers have gone wrong but the data is still there.
Tomorrow I will backup the one crucial VM onto a USB drive, and at that stage I will be in place to safely reinstall ESXi.
I'll keep you posted, but anyway, any suggestions to get my current host back on track would be welcome anyway.