Did you lock them in? I've noticed that their pathfinding AI isn't exactly perfect. more than once I've seen a villager outside a house jumping at the wall, like he's trying to fit inside the window. So... If they're allowed to roam free, then there's a good chance zombies picked them off one by while they were walking into walls at night like idiots.
Or is it in a desert? They still seem to ignore cactus and walk right into it..... --- http://dragcave.net/view/Yulg
My village is open to attacks and not very well lit and doesn't have a fence/wall preventing mobs out and keeping the villagers in, and I never had any problem with them getting killed off or disappearing. I think it's cool letting the villagers run around freely and not confining them in the town with fences. Although my village has like 60 or so doors on about 10-15 buildings each varying in size. They also spawn several iron golems to protect themselves, and they do a good job doing so.
Of course I am on Normal. On hard mode, I'd probably have trouble keeping the village alive without lighting the whole place up at least.
As for the TC, you either need to get new villagers from another village to start the process over again, or just move on. Also, try making your villages bigger so that they populate more and don't die out so quickly. --- Like it or leave it. Hey, where are you going?
Even if the zombies can't break down doors, the will still be attracted to them. The villagers, being completely brainless, are perfectly content to open their doors at sunrise no matter what might be standing outside. So the zombies walk up to the doors at night, the villagers open the doors in the morning and end up getting slaughtered. --- "Eh, the real world's ok. It's just hard to stay interested in something for decades at a time." -Lightning Bolt
Your best bet would be to make a zombie-proof playpen for your villagers until they learn that zombies are bad.
Just surround the village with a 2-block high wall with simple iron door entrances. Be sure to use buttons, as pressure plates can be used by zombies and you may forget about a lever. --- I have no signature.