Entities disappeared

When I was going to get some tropical fish for my Axolotls. I had to go travel in the Nether to get there and then when I got back my sheep, cows, some axolotls, villagers, and pets were all gone. I really spent a LONG TIME GETTING THE VILLAGERS IN THERE SMALL HUTS. And I really needed the sheep for beds to find Ancient Debris. Please fix this.