The post title pretty much says it all, but I'll expound:
Back up your files much more often than you think you need to. Because something will go wrong, and unless you have a backup copy of your file that is ideally stored in an entirely unrelated place (different hard drive, cloud storage if that's your preference, etc.) you could very well be SOL. All those things you think will save you (auto-save, manually saving every few minutes or every few changes, etc.) might very well basically laugh at you as you cry. In other words, they won't work. So I repeat: backing up your files, ideally somewhere other than where the main file is, might be the only thing that saves you.
If your time and work and heartache and tears matter to you, back up your file every single time you work with it.
Why am I writing about this? Because it turns out MS Word has a "known issue" where sometimes, just because it feels like it, it transforms your document into asterisks. The whole thing. Pages and pages and pages of asterisks. You want to see what my most recent version of G&H looks like? Here's a sneak peek:
Thankfully, I haven't lost the entire manuscript, just the most recent revisions, but honestly that is heartbreaking because I was finally on a roll with really fixing the current problems in the story. And now, I have to start over, and I just know the "how did I fix this last time?" version won't be as good.
So don't be me. Back up your files. Go now.