Dylan Thomas, the Welsh poet, had the only draft of his radio play Under Milk Wood with him when he went on a pub crawl. He didn’t have it with him when he returned, and given the extraordinary amount of alcohol he put away, he didn’t recall where he might have left it.
A BBC producer had to search pub after pub after pub, desperate to find the script before closing time, lest it be put out in the trash. He got lucky, very lucky, and recovered the play.
Back up your work and back it up again. Don’t depend solely on the cloud (I hear Dropbox is having issues lately) and go for offsite storage too.
This isn’t just for authors. We all know someone who’s suffered a data loss. Go back up your system.