No spinning mob in spawner

Zombie spawner was spawning fine and then I realize that zombies are not spawning anymore so I take a look at the spawner and there is flames, but when I look closer, there is no spinning zombie mob inside the spawner anymore, causing it to not spawn any zombies. I am playing in survival mode and have not switched to creative in this world. The only thing that I have done was update this morning for bug fixes and I have switched between peaceful and hard difficulty. The spawner is usually dark, I am just using the torches so you can see that there is nothing in the spawner.