I can't tell you how often (yes, still) people call me because, they've lost their data and never thought they'd really need a back-up. Sad, but true: most people think data loss won't ever happen to them. And when disaster strikes (and it eventually will), dealing with it is emotional, expensive and time-consuming. Also: you've JUST LOST YOUR DATA!
So don't wait to be REactive and instead be PROactive. That means you do something now, before the problem happens, right? Right. That means having a regular back-up. If you're on a Mac (I am), you can connect an external hard drive and use Time Machine which is free and included as part of the Macintosh OS. If you're on a PC, you can use Symantec System Recovery which is reasonably priced at around $90.
And, for those of you who are either "on the go" with your computer or simply don't like having to connect something physically to back up your data: then back-up to the internet. I use a great program called CrashPlan made by Code 42, but there are others like Carbonite or Mozy that all do the same thing. Most of these companies charge about $60/year for their unlimited back-up service, but It's worth it for the peace of mind and the time saved.