What happened to the windmill when it was half built?

During chapter 6 of Animal Farm, the windmill is slowly being built and the animals were very proud of the progress. However, disaster strikes when a nigh time storm destroys it.

During the animals sleep in the middle of the night, huge south-west winds tore at farm and (along with the elm tree being ripped out of the ground, the farm buildings rocked, tiles blown off of the roof) when the animals woke in the morning, they went out to examine the damage; though they stopped when they noticed the windmill was lying on the grounds in ruins. Suddenly Napoleon blamed Snowball for the whole thing.