World not saving

I spent about 45 minutes building a room for a storage area for my base. I left the world, and when I re-opened the world (without closing the game), I was at my previous spawnpoint. Even worse, every change I had made was completely deleted. Nothing was saved. I was fortunate enough to have accidentally taken a screenshot, so I have a before and after picture. The after picture is me standing on top of my base, viewing where the room once used to be. The before picture shows me standing in the room, partially completed.