Sheila,
When working with large or important documents I always:
1) Save it regularly with a different name as a back up.
So if you have a Word document called Story.doc, after you have done a save, do a separate save (into another directory) as Story01, then later Story02, then later Story03 etc.
Then exit Word, and reopen Story.doc and work on that.
That way you know you always have a recent backup of the whole document.
2) Also get into the habit of saving a large or important document onto another medium. Maybe a USB key, or a back CD or DVD.
Hard disks can and do go wrong and you can lose EVERYTHING on a hard disk.
If a file is backed up to a CD or DVD then you always have a copy to go back to.