Also, I suggest saving it periodically using a different name (Save As) so that you have multiple files that you have saved. You can always delete the redundant files later. I follow the convention of adding a number to the end of the file name, and incrementing it by one when I save a new version: for example, PA552paper1, PA552paper2, PA552paper3, etc.
Finally, I recommend always having copies of any important document in at least two different locations; for example, on your hard drive and on a CD or a USB drive.
If you follow these rules, like I do, you will not loose work again. Too many people loose their work when just taking some basic precautions like this will prevent such loss.