I'm gon' tell a lil story. I made an puzzle map before and it was huge. It would take at least 4 hours to complete it. Suddenly, when I was about to put my finishing touch on it, it just completely wiped out. What I'm trying to say is, you cannot PREDICT when it's gonna happen, just back up your worlds. If you don't, I promise you, you will eventually learn the hard way.
Same thing happened to my friends private server. One of his mates burned down possibly 60-70
houses and HUNDREDS of hours of work. The whole town was gone. He didn't have backup's because it was only us friends. sucks man, i know, but at least now u will use backups.