Ubuntu :: Loss Of Files Followed By Inability To Mount Drive?

We've had a headless server running Ubuntu for a few weeks now and it's been fantastic - we've been able to access it from all of our computers and it's making life so much easier having one central box to put everything in. Until yesterday, where we noticed that some directories on the 1TB HDD were empty that were previously full of files. Restarted and the directories are gone, leaving only the root directories. Restarted again and Ubuntu doesn't even recognise the disk.

We have Ubuntu installed on a 250GB HDD with the 1TB hard drive mounted on (as?) /srv and all of the data is on there. Before Ubuntu stopped recognising the disk it would appear in Disk Utility saying that the file system was corrupted and that there was an issue with the superblock - I've found some ways of attempting to fix that but none are working now that Ubuntu isn't able to find the disk. I've checked the physical connections and they all appear to be working fine. I can hear/feel the hard disk spinning. Some users haven't backed up the files they put on the server so I'd prefer to try anything non-destructive first and save their work. I'm hoping that the HDD hasn't totally died... but now that Ubuntu can't see it I'm concerned that that might be the case.