When this kind of thing happens, you can close the game, then go into My Documents->LDW->Virtual Villagers - The Lost Children, and open ldw.log. That file will tell you, more or less, what happened during the night.

"Wipes" where everyone dies do happen from time to time, but as you understand the game better, they become easier to avoid. Likely things: too many sick farmers, not enough designated healers, not enough farmers, or only female farmers, who got pregnant during the night.