This happened to during #CampNaNoWriMo; I was happily working on my novel when the little hated window of "register software" made it's unwanted appearance. Turns out a virus corrupted Office and somehow removed the serial key. That was my worst nightmare.
Although the more common bad experiences with PC's are files bring deleted, either accidentally or by virus. So to prevent your worst nightmare from ruining your progress here's what you can do:
-back up your WIP. Email your work to yourself. Don't trust memory sticks, I learned the hard way that they too can fail! Or use Dropbbox or Google Drive to back up your work It's safe and private and accessible from your smart phone even.
-Have back up software. If you got a backup of MS Office then great! But if you can't afford to fork out the cash then rather use Open Office. It's got all the functionality of Word and it's FREE. No worries about licences that suddenly expire.
-if it was accidental deletion you can always try recovery programs with some success.
-print a hardcopy. This sounds nuts, being in the digital age and all. Trust me on this, a hardcopy will save you when all of the above failed. And it makes editing easier too.
Well hope this helps. And may you never have to learn the hard way how important it is to back up your work. PC's fail. We tend to forget that.