Villagers Disappeared from Iron Farm (Survival)

Spent a couple of days, off and on, building an iron farm with a 50 bed villager breeder. Had probably got it up to around 20 villagers and iron has started coming through the collection system. After spending some time repairing tools and leveling at a skeleton spawner XP farm (very close to the iron farm laterally but some way below), I realised that the iron had stopped coming through and after going to investigate, all the villagers had gone. I used on the of the more popular designs from youtube and it was working well. The villager area was well lit and had a glass block roof (see attached screenshot). The missing blocks in the shot were taken out after the disappearance. How can I recover the villagers; it took me ages to get the two initial villagers into the farm?