You can avoid this. Here are four easy ways to backup your manuscript and avoid disaster. However, before I list them, you must keep two very important principles in mind:
Multiple versions: It is very important to maintain multiple older versions of your files. Why? Let's say a virus infected your computer and corrupts your manuscript file, but you don't know it yet. Then you backup your manuscript onto a USB drive, copying over the only other existing copy. Now both the original file and the backup file are corrupted. To counter-act this, you want a backup solution that maintains all (or at least many) of the older versions of your files. This way you can go as far back as needed to fine a working version of your file.
Multiple locations: If you backup to a hard drive in your home and have a flood, fire, or theft, you could lose both your computer and your backup location at once. Just having your data in the cloud (e.g. with Google Docs) isn't insurance against this, because they can have failures too. Ideally you want your files in multiple locations: In your home and somewhere on the internet at a minimum.
In order of ease of use and security, here are my recommendations:
1. Install backup softwareThis is the single best thing you can do, because once it's installed, you can forget about it, and it'll keep working for you. Do a quick google search for backup software, pick something, and install it. On the Mac, you can use the built-in Time Machine, but Super-Duper is popular as well. Lifehacker has several recommendations for Windows backup software.
Ease of setup: medium
Security: good to excellent (depending on where you back up: ideally it would include the cloud and an external drive)
2. Install DropboxDropbox is fantastic software for sharing files between computers and to the cloud. (Cloud meaning, in this case, their servers on the Internet.) I use Dropbox to share files between my personal computer and my work computer, and a copy of the files also reside on Dropbox's servers. This means that there are always at least three copies of my files. If you upgrade to the paid version of Dropbox, you can store all older revisions of your files.
Ease of setup: extremely easy
3. Email to yourself on GmailIf you have a Gmail account, which is effectively unlimited in size, you can easily email your entire manuscript to yourself as frequently as needed. Because the files are maintained on Google's servers, if anything happens to your computer, you can still access them from anywhere you can log into Gmail.
Ease: medium (you have to remember to do it)
Security: medium (again, you have to remember to do it)
4. Copy to USB driveYou can also copy your files to a USB drive. These are so cheap they're almost free. You can get a tiny (in size) flash drive, or a larger hard-drive based model that can backup your entire computer.
Ease: medium (you have to remember to do it, and you have to find/have the USB drive.)
Security: medium-poor (you can lose the drive, it can crash, etc.)
I hope that you'll do at least one, if not two of these methods. Otherwise you are putting yourself at risk for a loss of your manuscripts, and that is a very painful loss to experience.