It might have already happened to you, but if it hasn't don't wait for it to teach you a costly lesson: Always make backups ! Purchase an external drive, seperate from your computer's memory so that if it crashes, you can at least feel safe your personal data is safe.
Depending on the worth of your data, determin if you should backup every day, week or month, but make sure you do. Keep an alert notification that promps you to backup after certain periods of time if it helps. There are plenty of available utilities and applications online to help make an exact copy of your machine.
When buying a hard drive, look for something with a similar capacity to your own computer, so that you don't always have to leave stuff out from lack of space. If you have alot of data and no time to waste, invest in a hard drive with firewire or optic capabilities, since USB can be quite slow.
Once you store a copy of you data you must find somewhere to put it so that not only is it safe from being tampered with, but by the elements as well. I recommend buying a small fireproof and waterproof chest safe, in case of fire. These basic safes can be very affordable, the ones that cost a little more are those which provide stronger resistance against prying and breaking of the locks. If your only concern is fire, as well as the flood of water the firemen will send into your house, a small Sentry Safe is ideal. I go into further details on finding the right safe in this article I wrote.
An alternative way to disaster-proof you data is to store it online. Privacy is never 100% assured online, so please don't keep anything too sensitive. There are several websites providing this service but all at an monthly or annual cost. However, the advantage is that you can obtain Data Syncing, where your data is automatically updated as they are modified or otherwise changed.
And of course if you're very paranoid (like I am) about loosing your data, always make two backups. I have one drive I use for my weekly backups, the other I use once a month. I'd have to be the unluckiest person on the planet to have my computer and two hard drives crash on me at once. If unfortunatly, you're reading this because you've already lost your data, don't loose hope, there are many recovery services that will extract any remaining data on the drive.