After travelling for the American holiday, I returned home to find my iMac in an unbootable state. I'm not sure what happened exactly — all I could tell is that a hard drive error occurred around 3am one night, corrupting the system. Disk Utility, which often works magic, couldn't fix the error. But, not to fear, four hours later Time Machine had me back up and running exactly where I left off.
I can't encourage you enough to take a moment right now during this holiday weekend and make sure your backups are in place and working. (Make sure you test them, too! A backup you can't recover from does you no good.) And, if they're not, take my advice and set them up now.