Villagers have all disappeared

I had a large room of quite a few villagers, all of whom were assigned a bed, all surrounding a bell. I traded often with the villagers, (who I got to my house through the painstakingly slow land-boat), and was breeding them about once every in game week. 
I stopped playing for a few days, and when I returned to my world, every single villager was gone without a trace, despite there being no way for any of the villagers to be killed, since they were all in a underground bunker that couldn't be accessed by mobs. Now I have no villagers, and every village I find has yet to have any villagers in them.