Here’s the scenario: all is well, you’re getting lots of work done on your computer, when all of a sudden it crashes. No idea why — it just crashes.
What’s worse, it won’t reboot. Maybe it just hangs on boot, maybe it says the drive isn’t found, maybe there’s something else interfering.
To make matters worse yet, you stand to lose all that work you were doing, because it hasn’t yet been backed up. Your most recent work seems to be locked up inside a hard drive you can’t access.
There may be a way to back it up before dealing with the crash.