Server didn't save??

My server didn't save from when I last played yesterday, and I don't know why.
My friend and I got on to play for a bit and then I left, and he later left. Then later that day I got on again with my other friend and I played for a bit before I got off again, but she continued to play. Later on my first friend joined her and they played for the majority of the evening until they both logged off and I got on again to play for a few minutes.
I distinctly remember they had finished the majority of their houses, because I went to go check on one of my friends' mansion and she was almost done with hers.
Then I had to get off so I closed everything out, and now that I've gone into the world again, it's like none of us worked on it yesterday.
I've already tried clicking on the 'Restore Backup' button on the server page, but it said it didn't find a backup, and I'm pretty upset.
Does anyone know why it did this? Or how I can fix it?

Whenever I log off I always "Stop" the server.