I left my Linux box running overnight, and I came back the next morning to find this:
Does this mean my harddrive has crashed or is crashing? Or, is this likely caused by some other issue and easily fixable?
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It could. There's probably ways to check.
You'll want to start by preparing a USB key with a linux live distro. Xubuntu is probably a good start, or some other distro with a lightweight WM is a good idea.
gddrescue/ddrescue (the package on ubuntu is called gddrescue, but the command is ddrescue) should help you get as much data out as possible, and might make recovery possible. In your current state this should be the very first thing you do.
Do this first. You can mount the resulting drive image with kpartx, and run stuff like fsck (preferably on a copy of the drive image) to try to fix the drive contents.
You also want to check if the disk is actually damaged. gsmartcontrol is nice for that - it highlights any obvious issues.
Your grub install or your partition table is damaged, which had been caused by a software problem, too. You can reinstall your grub by booting from a live or rescue cd.
If your partition table is also damaged, you had to reconstruct it by tools designed for the task.
Watch for hardware failure signs in the kernel message logs (dmesg command).