Naturally spawned Zombie Village?

Solution 1:

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.

Solution 2:

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!