Naturally spawned Zombie Village?

From the Minecraft Wiki article on villages:

"2% of villages will generate as zombie villages. In such villages, all generated villagers are instead zombie villagers, and all doors and torches are missing. [...]"

There's also a chance for a village to be attacked by a zombie siege as explained in another answer, but that can only occur at midnight, and since it's your first day in the world that's not possible.

The villagers all becoming zombies is probably a consequence of a Zombie Siege event.

The Zombie siege event is only available each night and there is a 10% chance that a zombie siege will spawn.

Zombie villagers can result from a zombie attack. If any type of zombie kills a villager, there is a chance that the villager will transform into a zombie villager. (100% on Hard difficulty, 50% on Normal, 0% on Easy).

• More info on Zombie Siege
• More info on Zombie Villagers

I hope that helped!