Half of my score is gone!

I have spent HOURS over the past two weeks entering the score for Gershwin's "Rhapsody in Blue", working a couple of hours each day. There are 29 pages, and I was working on page 27, and suddenly when I looked back at something, I saw that the last saved version is missing over half of the beginning! Not only that, but the part that is still in the score now has some measures missing, and others are out of order. It's a nightmare! I have saved some versions along the way using different file names just in case, but they don't represent a good part of the early work, because this seems to have happened with the later file saves, and the error was already present. I didn't normally look all the way back to the start every time I worked on this.

I know there is NO way to recover what's lost, but if anyone has an idea why this happened, I would love to know, so as to maybe prevent it from happening again.

Thanks for all the hard work that developers put into Musescore to make this great program available for free!