Went to watch something from my fileserver, on the PS3, and it just sat there doing nothing. Pressed stop, which responded instantly and back to the file-browser, so tried again with the same result.
Tried to SSH to the fileserver, nothing, although it was still pingable. So, tried plugging a monitor and keyboard into it, with absolutely no luck. Power-cycling was the only option left, so tried that...
As soon as it powered back up again, the monitor flickered into life, and I got the BIOS screen, went through the normal checks but the ominous feeling started when I saw it confirm the presence of three working disks (there are four in there), which was compounded by seeing the nice friendly "you have no operating system installed" error-message.
Now, given that the server is RAID-5, I'd assumed that losing a disk would be fine, and it would still boot up in a degraded state with all of the data intact - unfortunately, using software-RAID, it doesn't look like this is the case!

So, I now have 3 options, which I'll be trying in order:
1) Boot the system up, with three working drives connected, using a Linux LiveCD on USB key, and see if it can see any data.
2) Replace the broken drive, and see if the software-RAID system rebuilds the array from the three working disks.
3) Write off 2.3TB of data as lost and irretrievable (unless I fancy paying silly money for a company to do it - I don't), and rebuild the system with the new disks I've bought today, ie. the upgrade I was planning to do anyway (but preferably without losing all of the data in the process).
Fingers crossed #1 or #2 work, then I can make a backup of the data, and then go onto #3 without losing the data...