Until you save your document and give it a name, Excel calls it "Book1".  If you see this in the title bar, your work has not been saved, and you should do so immediately.

It is easy to forget to do this.  However, if you DO forget this vital task, and for some mysterious reason your computer crashes, or there is a power glitch or whatever, you will lose ALL your work.

Do yourself a favour - save at the beginning.