I've no idea what I did to make this happen, which probably makes it even worse.

Last night when I booted, I got the following:

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Running DevFs daemon Started device management daemon v1.3.2.5 for /dev
unknown group: "video", defaulting to GID=0

**CRITICAL**: unknown class "dri" at line 80 in /etc/security/console.perms
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Loading default keymap: etc/rc.d/rc.sysinit: line 248: /dev/tty0: no such file or directory

fsck.ext2/dev/hda5:
The superblock could not be read or does not describe a correct ext2 filesystem. If the device is valid and it really contains an ext2 filesystem (and not swap or ufs or something else), then the superblock is corrupt, and you might try running e2fsck with an alternative superblock:
e2fsck -b 8193 <device>
: No such file or directory while trying to open /dev/hda5
Failed to check filesystem. Do you want to repair?

Doesn't matter if I answer yes or no, I get prompted for a root password and it wants me to do something. Not much I can do, the /dev/ directory seems to be practically empty, I can't access the other drives or anything. I tried running Partition Magic on the drive and it found an error with the superblock, but can't fix it. But browsing the drive from Partition Magic shows all the data correctly, as far as I can tell.

So, at least I've got a way of saving some of my data before reinstalling. But before I do I thought I'd ask here to see if there is a way to recover the drive, or at least if someone could tell me what I might have done so I don't do it again in the future. I've no idea what it could be...

Use you startup disk, you did make a startup disk. Then run fsck.ext2, it'll probably not do a thing but it's worth a try.

Put in the root username, and run fsck like it says, using an alternate superblock. The reason it appears to be empty is that the the data describing the directory is now corrupt. Replacing the superblock should repair that. Don't lose hope, it's not anything serious.

Well, I tried doing e2fsck -b 32768 ... but it needs a device, and there's no /dev/hd?? anything, so it won't run. Am I doing something wrong?

Also tried using a bootdisk, did the same there, got loads of errors that were fixed, apparently. Rebooted, and got the same thing as before.

Any other ideas?

You can get a low level format utility from the drives manufaturer. It can be dangerous (can ruin the drive) but it will go over every square inch of the drive and afterwards you won't have bad superblocks. Just a last resort though.