Deleted World I Spent hours on

I was playing on this world that I spent many hours on, with a slime farm and a large house, but once I logged back in after being away for about 2 hours the whole world was deleted and replaced with a world of the same seed. All of my work and blocks that I placed were gone and left with an untouched world. I made sure to save the world when I lost logged out, so I don't know why this happened.