I just had a phone call from someone who is leaving in a few hours for their vacation and their MacBook is now displaying something like this on the screen:
The dreaded question mark on the Mac display. The hard drive may be lost. Or maybe it's just a weird issue that can be fixed with a reboot/disk check. That's not the point. They said they may have some of their info backed up, but aren't sure. Luckily they're heading somewhere close to an Apple store for their holidays so they'll get great support there - but what about their data?
Say it with me: Computers are not perfect. Mac computers are not perfect. The hard drive in your computer will fail.
If you're on a Windows computer, I can't help you. I'm sure there's some sort of Backup wizard that you can use with only 10 clicks.