Disappearing Mobs

Hello,

I had 3 dogs, 32 sheep, 23 cows and 20 villagers in my survival world. Sheeps and cows for a food source and villagers in an iron farm. I know they are still there because I can hear them. I can hear many sheep ba-ahing and many cows mooing, my dogs woofing. And the horrible villager mumble. But they are INVISIBLE. They are just gone. the iron farm still works, but I would still like to see my dogs please. Please try your best to fix this problem.